diff --git a/.clinerules b/.clinerules
index e3ccaea..ea9179c 100644
--- a/.clinerules
+++ b/.clinerules
@@ -52,4 +52,6 @@ Each feature MUST contain:
 - Check code quality:
   flutter analyze
 - Run test suites:
-  flutter test
\ No newline at end of file
+  flutter test
+
+Before starting any task, read BUILD_STATE.md to determine the current milestone. Upon satisfying all implementation steps and verification commands, update the checkboxes in BUILD_STATE.md before prompting the user.
\ No newline at end of file
diff --git a/BUILD_STATE.md b/BUILD_STATE.md
new file mode 100644
index 0000000..d4144e3
--- /dev/null
+++ b/BUILD_STATE.md
@@ -0,0 +1,30 @@
+# Build Execution Ledger
+
+## Active Target
+- **Current Milestone:** Phase 3: Nutrition, Protein Target Tracking & HealthGateway Sync
+- **Status:** Phase 2 Complete
+
+## Phase Checklist
+- [x] Phase 1: Foundation & Identity (Passwordless SSO & Encrypted Drift DB)
+- [x] Phase 2: Medication Domain, Injection Rotation, Serum Modeling
+- [ ] Phase 3: Nutrition, Protein Target Tracking, HealthGateway Sync
+- [ ] Phase 4: Recipe Web Scraper Engine & Meal Planning Calendar
+- [ ] Phase 5: Pantry Delta Engine & Auto-Shopping Generator
+- [ ] Phase 6: Hardening, Native Desktop/Web Polish, CI/CD
+
+## Phase 2 Deliverables (2026-10-16)
+- [x] Domain: `DoseEntry` entity, 12-quadrant `InjectionSite` rotation model, `Glp1Compound` value object (Sema 168h / Tirze 120h / Lira 13h), `IMedicationRepository` contract.
+- [x] Domain: `CalculateSerumLevel` use case implementing `C(t) = Σ D_i · 0.5^((t − t_i)/t_half)`.
+- [x] Data: injection-history CRUD on `AppDatabase` + drift-backed `MedicationRepository` (record ↔ entity mapping).
+- [x] Presentation: `MedicationController` (Riverpod StateNotifier) with serum estimation, least-recently-used site rotation, and reminder scheduling; refactored `MedicationScreen` (serum card, rotation suggestion, interactive log dialog, history with delete).
+- [x] Platform: `INotificationService` / `PlatformNotificationService` (flutter_local_notifications 17.x; Android/iOS/macOS scheduled reminders, graceful no-op on Web/Windows/Linux); Android boot-receiver manifest entries; `timezone` dependency added.
+- [x] Regression fix: `SupabaseAuthService` now resolves its client lazily (baseline widget test no longer throws when Supabase is uninitialized).
+- [x] Tests: 33 new unit/widget-fake tests (PK calculator, rotation policy, repository w/ in-memory drift, controller); baseline smoke test green.
+
+## Post-Phase-2 Additions
+- [x] Development auth bypass (2026-10-16): debug-only "Continue Without Signing In" button on the login screen (`kDebugMode`-gated, in-memory `devAuthBypassProvider`) lets reviewers enter the app without a Supabase session; the router guard honors the flag and the shell shows an orange "Development mode - not signed in" banner while active. Covered by `test/features/auth/dev_auth_bypass_test.dart` (3 tests).
+
+## Active Verification Gates
+- [x] `flutter pub run build_runner build --delete-conflicting-outputs`
+- [x] `flutter analyze` (Zero issues)
+- [x] `flutter test` (37/37 passing)
\ No newline at end of file
diff --git a/android/app/src/main/AndroidManifest.xml b/android/app/src/main/AndroidManifest.xml
index d14d7df..edd5269 100644
--- a/android/app/src/main/AndroidManifest.xml
+++ b/android/app/src/main/AndroidManifest.xml
@@ -1,4 +1,8 @@
 <manifest xmlns:android="http://schemas.android.com/apk/res/android">
+    <!-- Local dose reminders (flutter_local_notifications): allow reboot-time
+         re-scheduling of pending injection reminders and notification display. -->
+    <uses-permission android:name="android.permission.RECEIVE_BOOT_COMPLETED"/>
+    <uses-permission android:name="android.permission.SCHEDULE_EXACT_ALARM"/>
     <application
         android:label="glpcompanion"
         android:name="${applicationName}"
@@ -30,6 +34,17 @@
         <meta-data
             android:name="flutterEmbedding"
             android:value="2" />
+        <!-- Restores pending dose reminders after a device reboot. -->
+        <receiver android:exported="false"
+            android:name="com.dexterous.flutterlocalnotifications.ScheduledNotificationReceiver" />
+        <receiver android:exported="false"
+            android:name="com.dexterous.flutterlocalnotifications.ScheduledNotificationBootReceiver">
+            <intent-filter>
+                <action android:name="android.intent.action.BOOT_COMPLETED"/>
+                <action android:name="android.intent.action.QUICKBOOT_POWERON" />
+                <action android:name="com.htc.intent.action.QUICKBOOT_POWERON"/>
+            </intent-filter>
+        </receiver>
     </application>
     <!-- Required to query activities that can process text, see:
          https://developer.android.com/training/package-visibility and
diff --git a/lib/app/router/app_router.dart b/lib/app/router/app_router.dart
index 2bd0ad5..07a09d1 100644
--- a/lib/app/router/app_router.dart
+++ b/lib/app/router/app_router.dart
@@ -5,6 +5,7 @@ import 'package:supabase_flutter/supabase_flutter.dart';
 
 import '../../../core/network/supabase_status.dart';
 import '../../features/auth/presentation/controllers/auth_controller.dart';
+import '../../features/auth/presentation/controllers/dev_auth_bypass.dart';
 import '../../features/auth/presentation/screens/login_screen.dart';
 import '../../features/medication/presentation/screens/medication_screen.dart';
 import '../../features/nutrition/presentation/screens/nutrition_screen.dart';
@@ -18,22 +19,20 @@ const Set<String> _publicPaths = {'/login'};
 final _rootNavigatorKey = GlobalKey<NavigatorState>();
 final _shellNavigatorKey = GlobalKey<NavigatorState>();
 
-/// Application router with a reactive authentication guard.
-///
-/// The guard re-evaluates on every GoTrue auth transition (bridged into
-/// [refreshListenable] below), so a successful magic-link or OAuth round trip
-/// immediately navigates into the protected shell, and signing out bounces the
-/// user back to `/login`.
+/// Application router with a reactive authentication guard and dev bypass hook.
 final routerProvider = Provider<GoRouter>((ref) {
   final authRefreshNotifier = ValueNotifier<int>(0);
   ref.onDispose(authRefreshNotifier.dispose);
+
+  // Trigger re-evaluation on GoTrue auth transitions or dev bypass toggle
   ref.listen(authStateProvider, (_, __) => authRefreshNotifier.value++);
+  ref.listen(devAuthBypassProvider, (_, __) => authRefreshNotifier.value++);
 
   return GoRouter(
     navigatorKey: _rootNavigatorKey,
     initialLocation: '/medication',
     refreshListenable: authRefreshNotifier,
-    redirect: _authGuard,
+    redirect: (context, state) => _authGuard(context, state, ref),
     routes: [
       GoRoute(
         path: '/login',
@@ -67,11 +66,21 @@ final routerProvider = Provider<GoRouter>((ref) {
   );
 });
 
-/// Redirect guard enforcing the passwordless session lifecycle:
+/// Redirect guard enforcing the passwordless session lifecycle with dev bypass.
+///
+/// * Development bypass (debug builds only): `/login` -> `/medication`, all
+///   other routes pass without a session.
 /// * Unauthenticated sessions on a protected route -> `/login`.
 /// * Authenticated sessions on `/login` -> `/medication`.
-String? _authGuard(BuildContext context, GoRouterState state) {
+String? _authGuard(BuildContext context, GoRouterState state, Ref ref) {
+  final isDevBypassed = ref.read(devAuthBypassProvider);
   final location = state.matchedLocation;
+
+  if (isDevBypassed) {
+    if (location == '/login') return '/medication';
+    return null;
+  }
+
   final isAuthenticated = isSupabaseAvailable &&
       Supabase.instance.client.auth.currentSession != null;
 
@@ -83,4 +92,3 @@ String? _authGuard(BuildContext context, GoRouterState state) {
   }
   return null;
 }
-
diff --git a/lib/app/scaffold/adaptive_home_scaffold.dart b/lib/app/scaffold/adaptive_home_scaffold.dart
index 44140ba..806fc69 100644
--- a/lib/app/scaffold/adaptive_home_scaffold.dart
+++ b/lib/app/scaffold/adaptive_home_scaffold.dart
@@ -1,7 +1,11 @@
+import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:go_router/go_router.dart';
 
-class AdaptiveHomeScaffold extends StatelessWidget {
+import '../../features/auth/presentation/controllers/dev_auth_bypass.dart';
+
+class AdaptiveHomeScaffold extends ConsumerWidget {
   final Widget child;
 
   const AdaptiveHomeScaffold({
@@ -36,51 +40,67 @@ class AdaptiveHomeScaffold extends StatelessWidget {
   }
 
   @override
-  Widget build(BuildContext context) {
+  Widget build(BuildContext context, WidgetRef ref) {
     final width = MediaQuery.of(context).size.width;
     final isDesktop = width >= 1024;
     final selectedIndex = _calculateSelectedIndex(context);
+    // Visual marker while the debug auth bypass is active, so a review
+    // session can never be mistaken for a signed-in one.
+    final showDevBanner = kDebugMode && ref.watch(devAuthBypassProvider);
 
     if (isDesktop) {
       return Scaffold(
-        body: Row(
+        body: Column(
           children: [
-            NavigationRail(
-              selectedIndex: selectedIndex,
-              onDestinationSelected: (idx) => _onItemTapped(idx, context),
-              labelType: NavigationRailLabelType.all,
-              destinations: const [
-                NavigationRailDestination(
-                  icon: Icon(Icons.medication_outlined),
-                  selectedIcon: Icon(Icons.medication),
-                  label: Text('Medication'),
-                ),
-                NavigationRailDestination(
-                  icon: Icon(Icons.restaurant_outlined),
-                  selectedIcon: Icon(Icons.restaurant),
-                  label: Text('Nutrition'),
-                ),
-                NavigationRailDestination(
-                  icon: Icon(Icons.calendar_month_outlined),
-                  selectedIcon: Icon(Icons.calendar_month),
-                  label: Text('Meal Plan'),
-                ),
-                NavigationRailDestination(
-                  icon: Icon(Icons.kitchen_outlined),
-                  selectedIcon: Icon(Icons.kitchen),
-                  label: Text('Pantry'),
-                ),
-              ],
+            if (showDevBanner) const _DevAccessBanner(),
+            Expanded(
+              child: Row(
+                children: [
+                  NavigationRail(
+                    selectedIndex: selectedIndex,
+                    onDestinationSelected: (idx) =>
+                        _onItemTapped(idx, context),
+                    labelType: NavigationRailLabelType.all,
+                    destinations: const [
+                      NavigationRailDestination(
+                        icon: Icon(Icons.medication_outlined),
+                        selectedIcon: Icon(Icons.medication),
+                        label: Text('Medication'),
+                      ),
+                      NavigationRailDestination(
+                        icon: Icon(Icons.restaurant_outlined),
+                        selectedIcon: Icon(Icons.restaurant),
+                        label: Text('Nutrition'),
+                      ),
+                      NavigationRailDestination(
+                        icon: Icon(Icons.calendar_month_outlined),
+                        selectedIcon: Icon(Icons.calendar_month),
+                        label: Text('Meal Plan'),
+                      ),
+                      NavigationRailDestination(
+                        icon: Icon(Icons.kitchen_outlined),
+                        selectedIcon: Icon(Icons.kitchen),
+                        label: Text('Pantry'),
+                      ),
+                    ],
+                  ),
+                  const VerticalDivider(thickness: 1, width: 1),
+                  Expanded(child: child),
+                ],
+              ),
             ),
-            const VerticalDivider(thickness: 1, width: 1),
-            Expanded(child: child),
           ],
         ),
       );
     }
 
     return Scaffold(
-      body: child,
+      body: Column(
+        children: [
+          if (showDevBanner) const _DevAccessBanner(),
+          Expanded(child: child),
+        ],
+      ),
       bottomNavigationBar: NavigationBar(
         selectedIndex: selectedIndex,
         onDestinationSelected: (idx) => _onItemTapped(idx, context),
@@ -110,3 +130,36 @@ class AdaptiveHomeScaffold extends StatelessWidget {
     );
   }
 }
+
+/// Slim status strip shown while the development auth bypass is active.
+class _DevAccessBanner extends StatelessWidget {
+  const _DevAccessBanner();
+
+  @override
+  Widget build(BuildContext context) {
+    return Material(
+      color: Colors.orange,
+      child: SafeArea(
+        bottom: false,
+        child: Padding(
+          padding: const EdgeInsets.symmetric(horizontal: 16, vertical: 6),
+          child: Row(
+            children: [
+              const Icon(Icons.bug_report_outlined,
+                  size: 16, color: Colors.white),
+              const SizedBox(width: 8),
+              Expanded(
+                child: Text(
+                  'Development mode - not signed in',
+                  style: Theme.of(context).textTheme.labelMedium?.copyWith(
+                        color: Colors.white,
+                      ),
+                ),
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+}
diff --git a/lib/core/database/app_database.dart b/lib/core/database/app_database.dart
index 85452f6..0ce7b6d 100644
--- a/lib/core/database/app_database.dart
+++ b/lib/core/database/app_database.dart
@@ -52,6 +52,70 @@ class AppDatabase extends _$AppDatabase {
 
   @override
   int get schemaVersion => 1;
+
+  // ---------------------------------------------------------------------------
+  // Injection history CRUD
+  // ---------------------------------------------------------------------------
+  //
+  // These helpers intentionally operate on drift records/companions only:
+  // the `core` layer never imports feature domain types, so mapping between
+  // `MedicationDoseRecord` and the medication feature's `DoseEntry` entity
+  // happens in `features/medication/data/repositories/medication_repository.dart`.
+
+  /// Emits the stored dose history, newest first, and re-emits on every write.
+  ///
+  /// Ordering uses the administered time (falling back to the scheduled time
+  /// for pending rows) so backdated entries sort chronologically.
+  Stream<List<MedicationDoseRecord>> watchDoseEntries({int limit = 500}) =>
+      (_selectDoseEntries()..limit(limit)).watch();
+
+  /// One-shot snapshot of the dose history, newest first.
+  Future<List<MedicationDoseRecord>> getDoseEntries({int limit = 500}) =>
+      (_selectDoseEntries()..limit(limit)).get();
+
+  /// Inserts a new row and returns the persisted record including its
+  /// auto-incremented primary key.
+  Future<MedicationDoseRecord> insertDoseEntry(
+    MedicationDosesTableCompanion entry,
+  ) async {
+    final id = await into(medicationDosesTable).insert(entry);
+    return (select(medicationDosesTable)..where((t) => t.id.equals(id)))
+        .getSingle();
+  }
+
+  /// Partially updates the row identified by [id] (only non-absent companion
+  /// columns are written).
+  Future<void> updateDoseEntry(
+    int id,
+    MedicationDosesTableCompanion updates,
+  ) =>
+      (update(medicationDosesTable)..where((t) => t.id.equals(id))).write(
+        updates,
+      );
+
+  /// Deletes the row identified by [id]; returns the number of removed rows.
+  Future<int> deleteDoseEntry(int id) =>
+      (delete(medicationDosesTable)..where((t) => t.id.equals(id))).go();
+
+  /// Most recent row that was actually administered, or `null` when no dose
+  /// has been logged yet.
+  Future<MedicationDoseRecord?> latestAdministeredDose() =>
+      (select(medicationDosesTable)
+            ..where((t) => t.administeredTime.isNotNull())
+            ..orderBy([
+              (t) => OrderingTerm.desc(t.administeredTime),
+              (t) => OrderingTerm.desc(t.id),
+            ])
+            ..limit(1))
+          .getSingleOrNull();
+
+  /// Shared newest-first ordering for [watchDoseEntries] and [getDoseEntries].
+  SimpleSelectStatement<$MedicationDosesTableTable, MedicationDoseRecord>
+      _selectDoseEntries() => select(medicationDosesTable)
+        ..orderBy([
+          (t) => OrderingTerm.desc(t.administeredTime),
+          (t) => OrderingTerm.desc(t.id),
+        ]);
 }
 
 /// Application-wide database instance; closed when the container is disposed.
diff --git a/lib/core/platform/notification_service.dart b/lib/core/platform/notification_service.dart
new file mode 100644
index 0000000..f460874
--- /dev/null
+++ b/lib/core/platform/notification_service.dart
@@ -0,0 +1,195 @@
+import 'dart:io';
+
+import 'package:flutter/foundation.dart';
+import 'package:flutter_local_notifications/flutter_local_notifications.dart';
+import 'package:timezone/data/latest.dart' as tz_data;
+import 'package:timezone/timezone.dart' as tz;
+
+/// Platform-agnostic contract for scheduling local (offline) reminders.
+///
+/// Presentation code depends only on this interface; the concrete
+/// [PlatformNotificationService] wraps `flutter_local_notifications` so the
+/// package never leaks into feature layers (architecture rule 1/2).
+abstract class INotificationService {
+  /// Prepares the underlying plugin. Safe to call repeatedly.
+  Future<void> initialize();
+
+  /// Requests OS-level notification permission.
+  ///
+  /// Returns `true` when alerts may be shown, `false` when the platform is
+  /// unsupported or the user declined.
+  Future<bool> requestPermissions();
+
+  /// Schedules a one-shot reminder at [scheduledAt] and returns whether the
+  /// platform accepted it. Times in the past are rejected.
+  Future<bool> scheduleDoseReminder({
+    required DateTime scheduledAt,
+    required String title,
+    required String body,
+    int id = kDoseReminderNotificationId,
+    String? payload,
+  });
+
+  /// Cancels the reminder registered under [id].
+  Future<void> cancelReminder(int id);
+
+  /// Cancels every pending reminder.
+  Future<void> cancelAllReminders();
+}
+
+/// Stable notification id for the recurring dose reminder slot; re-using one
+/// id replaces the previous scheduling instead of stacking duplicates.
+const int kDoseReminderNotificationId = 7401;
+
+/// `flutter_local_notifications`-backed implementation.
+///
+/// Support matrix (graceful degradation per architecture rule 3):
+/// * **Android / iOS / macOS:** full one-shot scheduling via `zonedSchedule`.
+/// * **Windows / Linux / Web:** scheduling is a logged no-op - the plugin has
+///   no scheduler on those targets yet, and the domain keeps functioning
+///   (in-app schedule view) without notifications.
+class PlatformNotificationService implements INotificationService {
+  PlatformNotificationService({FlutterLocalNotificationsPlugin? plugin})
+      : _plugin = plugin ?? FlutterLocalNotificationsPlugin();
+
+  static const String _channelId = 'medication_reminders';
+  static const String _channelName = 'Medication Reminders';
+  static const String _channelDescription =
+      'Dose reminders for GLP-1 injections and site rotation.';
+
+  static const NotificationDetails _notificationDetails = NotificationDetails(
+    android: AndroidNotificationDetails(
+      _channelId,
+      _channelName,
+      channelDescription: _channelDescription,
+      importance: Importance.max,
+      priority: Priority.high,
+    ),
+    iOS: DarwinNotificationDetails(),
+    macOS: DarwinNotificationDetails(),
+  );
+
+  final FlutterLocalNotificationsPlugin _plugin;
+  Future<void>? _initialization;
+
+  /// Targets where the plugin can schedule future notifications.
+  bool get _supportsScheduling =>
+      !kIsWeb && (Platform.isAndroid || Platform.isIOS || Platform.isMacOS);
+
+  @override
+  Future<void> initialize() => _initialization ??= _initialize();
+
+  Future<void> _initialize() async {
+    if (!_supportsScheduling) return;
+
+    // Time zones are initialized once; scheduling uses absolute UTC instants,
+    // which the OS converts to the device's local time zone for display.
+    tz_data.initializeTimeZones();
+
+    // Permissions are requested explicitly via [requestPermissions] instead
+    // of implicitly during initialize().
+    const settings = InitializationSettings(
+      android: AndroidInitializationSettings('@mipmap/ic_launcher'),
+      iOS: DarwinInitializationSettings(
+        requestAlertPermission: false,
+        requestBadgePermission: false,
+        requestSoundPermission: false,
+      ),
+      macOS: DarwinInitializationSettings(
+        requestAlertPermission: false,
+        requestBadgePermission: false,
+        requestSoundPermission: false,
+      ),
+    );
+
+    try {
+      await _plugin.initialize(
+        settings,
+        onDidReceiveNotificationResponse: (response) {
+          debugPrint('Notification tapped: ${response.payload}');
+        },
+      );
+    } catch (error) {
+      debugPrint('Notification initialization failed: $error');
+    }
+  }
+
+  @override
+  Future<bool> requestPermissions() async {
+    if (!_supportsScheduling) return false;
+    try {
+      await initialize();
+
+      if (Platform.isAndroid) {
+        return await _plugin
+                .resolvePlatformSpecificImplementation<
+                    AndroidFlutterLocalNotificationsPlugin>()
+                ?.requestNotificationsPermission() ??
+            false;
+      }
+      // iOS and macOS share the Darwin permission surface.
+      return await _plugin
+              .resolvePlatformSpecificImplementation<
+                  IOSFlutterLocalNotificationsPlugin>()
+              ?.requestPermissions(alert: true, badge: true, sound: true) ??
+          false;
+    } catch (error) {
+      debugPrint('Notification permission request failed: $error');
+      return false;
+    }
+  }
+
+  @override
+  Future<bool> scheduleDoseReminder({
+    required DateTime scheduledAt,
+    required String title,
+    required String body,
+    int id = kDoseReminderNotificationId,
+    String? payload,
+  }) async {
+    if (!_supportsScheduling) return false;
+    if (!scheduledAt.isAfter(DateTime.now())) return false;
+
+    try {
+      await initialize();
+      await _plugin.zonedSchedule(
+        id,
+        title,
+        body,
+        // Absolute instants; the tray renders them in device-local time.
+        tz.TZDateTime.from(scheduledAt.toUtc(), tz.UTC),
+        _notificationDetails,
+        uiLocalNotificationDateInterpretation:
+            UILocalNotificationDateInterpretation.absoluteTime,
+        androidScheduleMode: AndroidScheduleMode.inexactAllowWhileIdle,
+        payload: payload,
+      );
+      return true;
+    } catch (error) {
+      debugPrint('Dose reminder scheduling failed: $error');
+      return false;
+    }
+  }
+
+  @override
+  Future<void> cancelReminder(int id) async {
+    if (!_supportsScheduling) return;
+    try {
+      await initialize();
+      await _plugin.cancel(id);
+    } catch (error) {
+      debugPrint('Notification cancel failed: $error');
+    }
+  }
+
+  @override
+  Future<void> cancelAllReminders() async {
+    if (!_supportsScheduling) return;
+    try {
+      await initialize();
+      await _plugin.cancelAll();
+    } catch (error) {
+      debugPrint('Notification cancel-all failed: $error');
+    }
+  }
+}
\ No newline at end of file
diff --git a/lib/core/utils/date_time_utils.dart b/lib/core/utils/date_time_utils.dart
new file mode 100644
index 0000000..e9f5d8d
--- /dev/null
+++ b/lib/core/utils/date_time_utils.dart
@@ -0,0 +1,38 @@
+// Shared, framework-free date helpers.
+//
+// The project intentionally avoids `intl` to keep core utilities usable from
+// pure Dart domain code and tests; these formatters cover the app's display
+// needs with explicit, stable output.
+
+const List<String> _monthNames = [
+  'Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun',
+  'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec',
+];
+
+/// Formats [time] as e.g. `Oct 16, 2026 · 2:05 PM`.
+String formatDateTime(DateTime time) {
+  final local = time.toLocal();
+  final hour12 = local.hour % 12 == 0 ? 12 : local.hour % 12;
+  final minute = local.minute.toString().padLeft(2, '0');
+  final period = local.hour < 12 ? 'AM' : 'PM';
+  return '${_monthNames[local.month - 1]} ${local.day}, ${local.year}'
+      ' · $hour12:$minute $period';
+}
+
+/// Formats a coarse elapsed time relative to [now], e.g. `never`, `3 hours
+/// ago`, `5 days ago`.
+String formatRelativeTime(DateTime time, {DateTime? now}) {
+  final reference = now ?? DateTime.now();
+  final elapsed = reference.difference(time);
+
+  if (elapsed.isNegative) {
+    final ahead = time.difference(reference);
+    if (ahead.inDays >= 1) return 'in ${ahead.inDays} day(s)';
+    if (ahead.inHours >= 1) return 'in ${ahead.inHours} hour(s)';
+    return 'in a moment';
+  }
+  if (elapsed.inMinutes < 1) return 'just now';
+  if (elapsed.inHours < 1) return '${elapsed.inMinutes} minute(s) ago';
+  if (elapsed.inDays < 1) return '${elapsed.inHours} hour(s) ago';
+  return '${elapsed.inDays} day(s) ago';
+}
\ No newline at end of file
diff --git a/lib/features/auth/data/supabase_auth_service.dart b/lib/features/auth/data/supabase_auth_service.dart
index 247730b..0efae08 100644
--- a/lib/features/auth/data/supabase_auth_service.dart
+++ b/lib/features/auth/data/supabase_auth_service.dart
@@ -31,11 +31,14 @@ String resolveAuthRedirectUrl() {
 /// line with the platform's security posture.
 class SupabaseAuthService implements IAuthService {
   /// Creates the service. Pass [client] in tests to inject a mock; production
-  /// code resolves the global Supabase client lazily on first use.
-  SupabaseAuthService({SupabaseClient? client})
-      : _client = client ?? Supabase.instance.client;
+  /// code resolves the global Supabase client lazily on first use so merely
+  /// constructing the service (e.g. while building the login screen before
+  /// `Supabase.initialize` completed, or inside widget tests) never throws.
+  SupabaseAuthService({SupabaseClient? client}) : _clientOverride = client;
 
-  final SupabaseClient _client;
+  final SupabaseClient? _clientOverride;
+
+  SupabaseClient get _client => _clientOverride ?? Supabase.instance.client;
 
   @override
   Future<void> signInWithGoogle() => _signInWithOAuth(OAuthProvider.google);
diff --git a/lib/features/auth/presentation/controllers/dev_auth_bypass.dart b/lib/features/auth/presentation/controllers/dev_auth_bypass.dart
new file mode 100644
index 0000000..19ebaeb
--- /dev/null
+++ b/lib/features/auth/presentation/controllers/dev_auth_bypass.dart
@@ -0,0 +1,14 @@
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+/// Development-only authentication bypass flag.
+///
+/// When `true`, the router's auth guard treats the session as authenticated,
+/// letting reviewers exercise the app without a configured Supabase project
+/// or a federated sign-in round trip.
+///
+/// Safety properties:
+/// * The toggle button is only rendered in debug builds (`kDebugMode`), so
+///   release/store builds never expose it.
+/// * State is in-memory only: every cold start re-enables the guard, so a
+///   bypassed session can never persist into a fresh launch.
+final devAuthBypassProvider = StateProvider<bool>((ref) => false);
\ No newline at end of file
diff --git a/lib/features/auth/presentation/screens/login_screen.dart b/lib/features/auth/presentation/screens/login_screen.dart
index 45eaace..cc5fb67 100644
--- a/lib/features/auth/presentation/screens/login_screen.dart
+++ b/lib/features/auth/presentation/screens/login_screen.dart
@@ -1,8 +1,10 @@
+import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:supabase_flutter/supabase_flutter.dart' show AuthException;
 
 import '../controllers/auth_controller.dart';
+import '../controllers/dev_auth_bypass.dart';
 
 /// Which authentication flow is currently executing, so the relevant button
 /// can render a spinner while the request is in flight.
@@ -115,6 +117,36 @@ class _LoginScreenState extends ConsumerState<LoginScreen> {
                           )
                         : const Text('Send Magic Link'),
                   ),
+                  if (kDebugMode) ...[
+                    const SizedBox(height: 24),
+                    const Row(
+                      children: [
+                        Expanded(child: Divider()),
+                        Padding(
+                          padding: EdgeInsets.symmetric(horizontal: 8.0),
+                          child: Text('DEV'),
+                        ),
+                        Expanded(child: Divider()),
+                      ],
+                    ),
+                    const SizedBox(height: 16),
+                    OutlinedButton.icon(
+                      onPressed: _continueInDevMode,
+                      style: OutlinedButton.styleFrom(
+                        foregroundColor: Colors.orange,
+                      ),
+                      icon: const Icon(Icons.bug_report_outlined),
+                      label: const Text('Continue Without Signing In'),
+                    ),
+                    const SizedBox(height: 8),
+                    const Text(
+                      'Development build only: skips authentication so the '
+                      'app can be reviewed without a Supabase project. Not '
+                      'available in release builds.',
+                      textAlign: TextAlign.center,
+                      style: TextStyle(fontSize: 12, color: Colors.grey),
+                    ),
+                  ],
                 ],
               ),
             ),
@@ -177,6 +209,13 @@ class _LoginScreenState extends ConsumerState<LoginScreen> {
     }
   }
 
+  /// Dev-only escape hatch: flips the bypass flag so the router's auth guard
+  /// admits the session without a Supabase round trip. The guard re-evaluates
+  /// through `refreshListenable`, which navigates away from `/login`.
+  void _continueInDevMode() {
+    ref.read(devAuthBypassProvider.notifier).state = true;
+  }
+
   Future<void> _sendMagicLink() async {
     final email = _emailController.text.trim();
     if (!_emailPattern.hasMatch(email)) {
diff --git a/lib/features/medication/data/repositories/medication_repository.dart b/lib/features/medication/data/repositories/medication_repository.dart
new file mode 100644
index 0000000..0ec9c27
--- /dev/null
+++ b/lib/features/medication/data/repositories/medication_repository.dart
@@ -0,0 +1,77 @@
+import 'package:drift/drift.dart';
+
+import '../../../../core/database/app_database.dart';
+import '../../domain/entities/dose_entry.dart';
+import '../../domain/entities/injection_site.dart';
+import '../../domain/repositories/medication_repository.dart';
+
+/// Drift-backed implementation of [IMedicationRepository].
+///
+/// Owns the translation between persistence records
+/// ([MedicationDoseRecord]) and the pure-Dart [DoseEntry] domain entity. The
+/// rotation quadrant is persisted as its stable enum name string so the
+/// database schema stays decoupled from domain types.
+class MedicationRepository implements IMedicationRepository {
+  MedicationRepository(this._database);
+
+  final AppDatabase _database;
+
+  @override
+  Stream<List<DoseEntry>> watchDoseEntries({int limit = 500}) => _database
+      .watchDoseEntries(limit: limit)
+      .map((records) => records.map(_mapRecord).toList(growable: false));
+
+  @override
+  Future<List<DoseEntry>> getDoseEntries({int limit = 500}) async =>
+      (await _database.getDoseEntries(limit: limit))
+          .map(_mapRecord)
+          .toList(growable: false);
+
+  @override
+  Future<DoseEntry> saveDoseEntry(DoseEntry entry) async {
+    final companion = _toCompanion(entry);
+
+    final MedicationDoseRecord record;
+    if (entry.id == null) {
+      record = await _database.insertDoseEntry(companion);
+    } else {
+      await _database.updateDoseEntry(entry.id!, companion);
+      record = await (_database.select(_database.medicationDosesTable)
+            ..where((t) => t.id.equals(entry.id!)))
+          .getSingle();
+    }
+    return _mapRecord(record);
+  }
+
+  @override
+  Future<void> deleteDoseEntry(int id) => _database.deleteDoseEntry(id);
+
+  @override
+  Future<DoseEntry?> getLatestAdministeredDose() async {
+    final record = await _database.latestAdministeredDose();
+    return record == null ? null : _mapRecord(record);
+  }
+
+  /// Converts a persistence record into the domain entity.
+  DoseEntry _mapRecord(MedicationDoseRecord record) => DoseEntry(
+        id: record.id,
+        compoundName: record.compoundName,
+        doseMg: record.doseMg,
+        administeredAt: record.administeredTime ?? record.scheduledTime,
+        scheduledAt: record.scheduledTime,
+        injectionSite: InjectionSite.tryParse(record.injectionSite),
+        notes: record.notes,
+      );
+
+  /// Converts a domain entity into an insert/update companion.
+  MedicationDosesTableCompanion _toCompanion(DoseEntry entry) =>
+      MedicationDosesTableCompanion(
+        id: entry.id == null ? const Value.absent() : Value(entry.id!),
+        compoundName: Value(entry.compoundName),
+        doseMg: Value(entry.doseMg),
+        scheduledTime: Value(entry.scheduledAt),
+        administeredTime: Value(entry.administeredAt),
+        injectionSite: Value(entry.injectionSite?.persistenceKey),
+        notes: Value(entry.notes),
+      );
+}
\ No newline at end of file
diff --git a/lib/features/medication/domain/entities/dose_entry.dart b/lib/features/medication/domain/entities/dose_entry.dart
new file mode 100644
index 0000000..1b790f9
--- /dev/null
+++ b/lib/features/medication/domain/entities/dose_entry.dart
@@ -0,0 +1,46 @@
+import 'injection_site.dart';
+
+/// Immutable record of a single GLP-1 administration event.
+///
+/// The entity is pure Dart (no framework imports) so it can be shared by the
+/// pharmacokinetic use cases, the drift-backed repository, and the Riverpod
+/// presentation layer without leaking infrastructure concerns into the
+/// domain.
+class DoseEntry {
+  DoseEntry({
+    this.id,
+    required this.compoundName,
+    required this.doseMg,
+    required this.administeredAt,
+    DateTime? scheduledAt,
+    this.injectionSite,
+    this.notes,
+  }) : scheduledAt = scheduledAt ?? administeredAt;
+
+  /// Database primary key; `null` until the entry has been persisted.
+  final int? id;
+
+  /// Commercial or compounded agonist name (e.g. `Tirzepatide`).
+  final String compoundName;
+
+  /// Total injected quantity in milligrams.
+  final double doseMg;
+
+  /// When the dose was actually administered. Drives the pharmacokinetic
+  /// decay calculation (`t_i` in C(t)).
+  final DateTime administeredAt;
+
+  /// Titration-schedule anchor the dose was prescribed for. Defaults to
+  /// [administeredAt] for immediately logged injections.
+  final DateTime scheduledAt;
+
+  /// Rotation quadrant used for this injection; `null` for legacy rows that
+  /// predate site tracking.
+  final InjectionSite? injectionSite;
+
+  /// Free-form clinician/patient notes.
+  final String? notes;
+
+  /// Whether a rotation quadrant was recorded.
+  bool get hasInjectionSite => injectionSite != null;
+}
\ No newline at end of file
diff --git a/lib/features/medication/domain/entities/injection_site.dart b/lib/features/medication/domain/entities/injection_site.dart
new file mode 100644
index 0000000..9306218
--- /dev/null
+++ b/lib/features/medication/domain/entities/injection_site.dart
@@ -0,0 +1,130 @@
+/// Anatomical regions targeted by subcutaneous GLP-1/GIP injections.
+enum InjectionBodyRegion {
+  abdomen('Abdomen'),
+  thigh('Thigh'),
+  upperArm('Upper Arm');
+
+  const InjectionBodyRegion(this.displayName);
+
+  final String displayName;
+}
+
+/// The 12-quadrant anatomical rotation matrix used to prevent
+/// lipohypertrophy and bruising at injection sites.
+///
+/// Quadrants are grouped into three regions of four sites each:
+/// * **Abdomen (4):** Right/Left x Upper/Lower
+/// * **Thighs (4):** Right/Left x Anterior/Lateral
+/// * **Upper arms (4):** Right/Left x Tricep/Lateral
+///
+/// Enum value names double as stable persistence identifiers (stored as text
+/// in the drift schema), so renaming a value is a breaking migration - add
+/// new quadrants instead of editing existing ones.
+enum InjectionSite {
+  abdomenRightUpper(
+    'Right Upper Abdomen',
+    'Right Upper',
+    InjectionBodyRegion.abdomen,
+  ),
+  abdomenRightLower(
+    'Right Lower Abdomen',
+    'Right Lower',
+    InjectionBodyRegion.abdomen,
+  ),
+  abdomenLeftUpper(
+    'Left Upper Abdomen',
+    'Left Upper',
+    InjectionBodyRegion.abdomen,
+  ),
+  abdomenLeftLower(
+    'Left Lower Abdomen',
+    'Left Lower',
+    InjectionBodyRegion.abdomen,
+  ),
+  thighRightAnterior(
+    'Right Anterior Thigh',
+    'Right Anterior',
+    InjectionBodyRegion.thigh,
+  ),
+  thighRightLateral(
+    'Right Lateral Thigh',
+    'Right Lateral',
+    InjectionBodyRegion.thigh,
+  ),
+  thighLeftAnterior(
+    'Left Anterior Thigh',
+    'Left Anterior',
+    InjectionBodyRegion.thigh,
+  ),
+  thighLeftLateral(
+    'Left Lateral Thigh',
+    'Left Lateral',
+    InjectionBodyRegion.thigh,
+  ),
+  armRightTricep('Right Tricep', 'Right Tricep', InjectionBodyRegion.upperArm),
+  armRightLateral(
+    'Right Lateral Arm',
+    'Right Lateral',
+    InjectionBodyRegion.upperArm,
+  ),
+  armLeftTricep('Left Tricep', 'Left Tricep', InjectionBodyRegion.upperArm),
+  armLeftLateral(
+    'Left Lateral Arm',
+    'Left Lateral',
+    InjectionBodyRegion.upperArm,
+  );
+
+  const InjectionSite(this.displayName, this.shortLabel, this.region);
+
+  /// Human-readable anatomical name (e.g. `Right Upper Abdomen`).
+  final String displayName;
+
+  /// Abbreviated label used inside compact pickers and chips.
+  final String shortLabel;
+
+  /// Region this quadrant belongs to.
+  final InjectionBodyRegion region;
+
+  /// Stable string persisted to (and parsed from) the database.
+  String get persistenceKey => name;
+
+  /// Parses a persisted identifier or display name back into a quadrant.
+  ///
+  /// Returns `null` for absent or unrecognized values so legacy rows with an
+  /// unknown site never crash the presentation layer.
+  static InjectionSite? tryParse(String? raw) {
+    if (raw == null || raw.trim().isEmpty) return null;
+    final normalized = raw.trim();
+    for (final site in values) {
+      if (site.name == normalized || site.displayName == normalized) {
+        return site;
+      }
+    }
+    return null;
+  }
+
+  /// Rotation policy: returns the least-recently-used quadrant.
+  ///
+  /// [lastUsedAt] maps a quadrant to the timestamp of its most recent use;
+  /// quadrants missing from the map (or mapped to `null`) count as never used
+  /// and therefore win over any previously used site. Ties - including ties
+  /// between never-used quadrants - resolve to the earliest entry in the
+  /// enum's declaration order, which keeps suggestions deterministic.
+  static InjectionSite leastRecentlyUsed(
+    Map<InjectionSite, DateTime?> lastUsedAt,
+  ) {
+    final never = DateTime.fromMillisecondsSinceEpoch(0);
+    var best = values.first;
+    var bestTime = lastUsedAt[best] ?? never;
+
+    for (final site in values.skip(1)) {
+      final siteTime = lastUsedAt[site] ?? never;
+      // Strict comparison preserves declaration order on ties.
+      if (siteTime.isBefore(bestTime)) {
+        best = site;
+        bestTime = siteTime;
+      }
+    }
+    return best;
+  }
+}
\ No newline at end of file
diff --git a/lib/features/medication/domain/entities/medication_dose.dart b/lib/features/medication/domain/entities/medication_dose.dart
deleted file mode 100644
index 46239ab..0000000
--- a/lib/features/medication/domain/entities/medication_dose.dart
+++ /dev/null
@@ -1,32 +0,0 @@
-enum InjectionQuadrant {
-  abdomenRightUpper,
-  abdomenRightLower,
-  abdomenLeftUpper,
-  abdomenLeftLower,
-  thighRight,
-  thighLeft,
-  armRight,
-  armLeft,
-}
-
-class MedicationDose {
-  final String id;
-  final String compoundName;
-  final double doseMg;
-  final DateTime scheduledTime;
-  final DateTime? administeredTime;
-  final InjectionQuadrant? injectionSite;
-  final String? notes;
-
-  const MedicationDose({
-    required this.id,
-    required this.compoundName,
-    required this.doseMg,
-    required this.scheduledTime,
-    this.administeredTime,
-    this.injectionSite,
-    this.notes,
-  });
-
-  bool get isCompleted => administeredTime != null;
-}
diff --git a/lib/features/medication/domain/repositories/medication_repository.dart b/lib/features/medication/domain/repositories/medication_repository.dart
new file mode 100644
index 0000000..89e1c29
--- /dev/null
+++ b/lib/features/medication/domain/repositories/medication_repository.dart
@@ -0,0 +1,25 @@
+import '../entities/dose_entry.dart';
+
+/// Persistence contract for the medication bounded context.
+///
+/// The presentation layer depends only on this interface; the drift-backed
+/// implementation lives in `data/repositories/` so widgets and controllers
+/// remain testable with in-memory fakes.
+abstract class IMedicationRepository {
+  /// Emits the full dose history (newest first) whenever it changes.
+  Stream<List<DoseEntry>> watchDoseEntries({int limit = 500});
+
+  /// One-shot snapshot of the dose history (newest first).
+  Future<List<DoseEntry>> getDoseEntries({int limit = 500});
+
+  /// Persists [entry]; when [DoseEntry.id] is `null` a new row is inserted,
+  /// otherwise the existing row is updated. Returns the stored entry with its
+  /// database-assigned identifier.
+  Future<DoseEntry> saveDoseEntry(DoseEntry entry);
+
+  /// Removes the entry with [id], if present.
+  Future<void> deleteDoseEntry(int id);
+
+  /// Most recent actually-administered dose, or `null` on an empty history.
+  Future<DoseEntry?> getLatestAdministeredDose();
+}
\ No newline at end of file
diff --git a/lib/features/medication/domain/usecases/calculate_serum_level.dart b/lib/features/medication/domain/usecases/calculate_serum_level.dart
new file mode 100644
index 0000000..4d6f54b
--- /dev/null
+++ b/lib/features/medication/domain/usecases/calculate_serum_level.dart
@@ -0,0 +1,69 @@
+import 'dart:math' as math;
+
+import '../entities/dose_entry.dart';
+import '../value_objects/glp1_compound.dart';
+
+/// Evaluates the accumulated pharmacokinetic serum concentration of GLP-1
+/// receptor agonists using the standard single-compartment open decay model:
+///
+/// ```text
+/// C(t) = sum over i of D_i * (0.5) ^ ((t - t_i) / t_half)
+/// ```
+///
+/// Where `D_i` is a dose administered at time `t_i` and `t_half` is the
+/// compound's elimination half-life (Semaglutide 168h, Tirzepatide 120h,
+/// Liraglutide 13h). Doses recorded with a future [DoseEntry.administeredAt]
+/// are ignored, and unrecognized compounds fall back to
+/// [defaultHalfLifeHours].
+///
+/// The use case is pure Dart with no dependencies, so it runs identically in
+/// the domain layer, widget tests, and unit tests.
+class CalculateSerumLevel {
+  const CalculateSerumLevel({this.defaultHalfLifeHours = 168});
+
+  /// Half-life assumed for compounds that are not in [Glp1Compound].
+  final double defaultHalfLifeHours;
+
+  /// Convenience entry point mirroring the mathematical notation
+  /// `C(t = [at])`.
+  double call({
+    required Iterable<DoseEntry> doses,
+    required DateTime at,
+    double? halfLifeHours,
+  }) =>
+      evaluate(doses: doses, at: at, halfLifeHours: halfLifeHours);
+
+  /// Computes the total active serum concentration (mg) at [at].
+  ///
+  /// [halfLifeHours] optionally forces a single half-life for every dose,
+  /// bypassing per-compound resolution (useful for custom titration studies).
+  double evaluate({
+    required Iterable<DoseEntry> doses,
+    required DateTime at,
+    double? halfLifeHours,
+  }) {
+    var totalMg = 0.0;
+
+    for (final dose in doses) {
+      if (dose.doseMg <= 0) continue;
+
+      final elapsed = at.difference(dose.administeredAt);
+      // Doses scheduled in the future have not entered circulation yet.
+      if (elapsed.isNegative) continue;
+
+      final halfLife = halfLifeHours ?? _halfLifeFor(dose.compoundName);
+      if (halfLife <= 0) continue;
+
+      final elapsedHours = elapsed.inMilliseconds / Duration.millisecondsPerHour;
+      totalMg += dose.doseMg * math.pow(0.5, elapsedHours / halfLife).toDouble();
+    }
+
+    return totalMg;
+  }
+
+  /// Resolves the elimination half-life for a compound label, falling back to
+  /// [defaultHalfLifeHours] when the name is unknown.
+  double _halfLifeFor(String compoundName) =>
+      Glp1Compound.tryParse(compoundName)?.halfLifeHours ??
+      defaultHalfLifeHours;
+}
\ No newline at end of file
diff --git a/lib/features/medication/domain/value_objects/glp1_compound.dart b/lib/features/medication/domain/value_objects/glp1_compound.dart
new file mode 100644
index 0000000..a4b3049
--- /dev/null
+++ b/lib/features/medication/domain/value_objects/glp1_compound.dart
@@ -0,0 +1,80 @@
+/// Supported GLP-1/GIP receptor agonists and their pharmacokinetic constants.
+///
+/// Half-lives follow the single-compartment decay values published in the
+/// prescribing information:
+/// * Semaglutide (Ozempic / Wegovy): ~168 hours (7 days)
+/// * Tirzepatide (Mounjaro / Zepbound): ~120 hours (5 days)
+/// * Liraglutide (Saxenda / Victoza): ~13 hours
+enum Glp1Compound {
+  semaglutide(
+    displayName: 'Semaglutide',
+    brandNames: 'Ozempic / Wegovy / Rybelsus',
+    halfLifeHours: 168,
+    dosingInterval: Duration(hours: 168),
+    defaultDoseMg: 0.25,
+    aliases: ['semaglutide', 'ozempic', 'wegovy', 'rybelsus'],
+  ),
+  tirzepatide(
+    displayName: 'Tirzepatide',
+    brandNames: 'Mounjaro / Zepbound',
+    halfLifeHours: 120,
+    dosingInterval: Duration(hours: 168),
+    defaultDoseMg: 2.5,
+    aliases: ['tirzepatide', 'mounjaro', 'zepbound'],
+  ),
+  liraglutide(
+    displayName: 'Liraglutide',
+    brandNames: 'Saxenda / Victoza',
+    halfLifeHours: 13,
+    dosingInterval: Duration(hours: 24),
+    defaultDoseMg: 0.6,
+    aliases: ['liraglutide', 'saxenda', 'victoza'],
+  );
+
+  const Glp1Compound({
+    required this.displayName,
+    required this.brandNames,
+    required this.halfLifeHours,
+    required this.dosingInterval,
+    required this.defaultDoseMg,
+    required this.aliases,
+  });
+
+  /// Generic compound name (e.g. `Tirzepatide`).
+  final String displayName;
+
+  /// Commercial brand names surfaced in the logging UI.
+  final String brandNames;
+
+  /// Elimination half-life `t_1/2` in hours, used by the pharmacokinetic
+  /// serum-level calculator.
+  final double halfLifeHours;
+
+  /// Recommended interval between administrations (dose reminder cadence).
+  final Duration dosingInterval;
+
+  /// Titration starting dose used to pre-fill the logging form.
+  final double defaultDoseMg;
+
+  /// Lowercase names recognized when parsing free-text compound labels.
+  final List<String> aliases;
+
+  /// Resolves a compound from a user- or database-supplied name.
+  ///
+  /// Matching is case-insensitive and accepts generic names, brand names, and
+  /// compound strings that merely contain an alias (e.g. `Semaglutide
+  /// (compounded)`). Returns `null` for unrecognized compounds so callers can
+  /// fall back to a default half-life.
+  static Glp1Compound? tryParse(String? raw) {
+    if (raw == null || raw.trim().isEmpty) return null;
+    final normalized = raw.toLowerCase();
+
+    for (final compound in values) {
+      if (compound.aliases.contains(normalized)) return compound;
+    }
+    for (final compound in values) {
+      if (compound.aliases.any(normalized.contains)) return compound;
+    }
+    return null;
+  }
+}
\ No newline at end of file
diff --git a/lib/features/medication/presentation/controllers/medication_controller.dart b/lib/features/medication/presentation/controllers/medication_controller.dart
new file mode 100644
index 0000000..e572ada
--- /dev/null
+++ b/lib/features/medication/presentation/controllers/medication_controller.dart
@@ -0,0 +1,273 @@
+import 'dart:async';
+
+import 'package:flutter/foundation.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import '../../../../core/database/app_database.dart';
+import '../../../../core/platform/notification_service.dart';
+import '../../data/repositories/medication_repository.dart';
+import '../../domain/entities/dose_entry.dart';
+import '../../domain/entities/injection_site.dart';
+import '../../domain/repositories/medication_repository.dart';
+import '../../domain/usecases/calculate_serum_level.dart';
+import '../../domain/value_objects/glp1_compound.dart';
+
+/// Application-wide injection history repository (drift-backed).
+///
+/// Overridable in widget/unit tests with an in-memory fake.
+final medicationRepositoryProvider = Provider<IMedicationRepository>(
+  (ref) => MedicationRepository(ref.watch(appDatabaseProvider)),
+);
+
+/// Cross-platform local reminder scheduler.
+///
+/// Overridable in tests with a recording fake.
+final notificationServiceProvider = Provider<INotificationService>(
+  (ref) => PlatformNotificationService(),
+);
+
+/// Immutable snapshot of the medication tracking state.
+class MedicationState {
+  const MedicationState({
+    required this.entries,
+    required this.isLoading,
+    required this.evaluatedAt,
+    required this.serumLevelMg,
+    required this.suggestedSite,
+    this.error,
+  });
+
+  /// Initial state while the first history snapshot loads.
+  factory MedicationState.initial() {
+    final now = DateTime.now();
+    return MedicationState(
+      entries: const [],
+      isLoading: true,
+      evaluatedAt: now,
+      serumLevelMg: 0,
+      suggestedSite: InjectionSite.leastRecentlyUsed(const {}),
+    );
+  }
+
+  /// Full dose history, newest first.
+  final List<DoseEntry> entries;
+
+  /// Whether the initial history load is still in flight.
+  final bool isLoading;
+
+  /// Most recent persistence/domain failure, cleared once data arrives.
+  final Object? error;
+
+  /// Timestamp at which [serumLevelMg] was evaluated.
+  final DateTime evaluatedAt;
+
+  /// Estimated active serum concentration (mg) at [evaluatedAt].
+  final double serumLevelMg;
+
+  /// Least-recently-used quadrant recommended for the next injection.
+  final InjectionSite suggestedSite;
+
+  /// Most recent administration, if any.
+  DoseEntry? get latestDose => entries.isEmpty ? null : entries.first;
+
+  /// Last time [site] was actually used, or `null` when never used.
+  DateTime? lastUsedAt(InjectionSite site) {
+    DateTime? latest;
+    for (final entry in entries) {
+      if (entry.injectionSite != site) continue;
+      final administeredAt = entry.administeredAt;
+      if (latest == null || administeredAt.isAfter(latest)) {
+        latest = administeredAt;
+      }
+    }
+    return latest;
+  }
+
+  /// When the next dose is due, derived from the latest administration plus
+  /// the compound's recommended dosing interval.
+  DateTime? get nextDoseDueAt {
+    final latest = latestDose;
+    if (latest == null) return null;
+    final interval =
+        Glp1Compound.tryParse(latest.compoundName)?.dosingInterval ??
+            const Duration(hours: 24);
+    return latest.administeredAt.add(interval);
+  }
+
+  MedicationState copyWith({
+    List<DoseEntry>? entries,
+    bool? isLoading,
+    Object? error,
+    DateTime? evaluatedAt,
+    double? serumLevelMg,
+    InjectionSite? suggestedSite,
+  }) {
+    return MedicationState(
+      entries: entries ?? this.entries,
+      isLoading: isLoading ?? this.isLoading,
+      error: error ?? this.error,
+      evaluatedAt: evaluatedAt ?? this.evaluatedAt,
+      serumLevelMg: serumLevelMg ?? this.serumLevelMg,
+      suggestedSite: suggestedSite ?? this.suggestedSite,
+    );
+  }
+}
+
+/// Orchestrates the medication feature: persists injections, derives the
+/// pharmacokinetic serum level, drives the injection-site rotation policy,
+/// and schedules the next local dose reminder.
+class MedicationController extends StateNotifier<MedicationState> {
+  MedicationController(
+    this._repository,
+    this._notifications, {
+    CalculateSerumLevel calculator = const CalculateSerumLevel(),
+  })  : _calculator = calculator,
+        super(MedicationState.initial()) {
+    _subscribe();
+  }
+
+  final IMedicationRepository _repository;
+  final INotificationService _notifications;
+  final CalculateSerumLevel _calculator;
+  StreamSubscription<List<DoseEntry>>? _subscription;
+
+  /// Re-attaches the history stream (used by the error-state retry action).
+  void retry() {
+    state = state.copyWith(isLoading: true, error: null);
+    _subscribe();
+  }
+
+  /// Recomputes the serum concentration for the current history at [now];
+  /// the screen calls this on a periodic ticker so the decay stays fresh.
+  void refreshSerumLevel({DateTime? now}) =>
+      _reduce(state.entries, now ?? DateTime.now());
+
+  /// Persists a new injection, refreshes derived state, and schedules the
+  /// next dose reminder. Returns whether the injection was stored.
+  Future<bool> logInjection({
+    required String compoundName,
+    required double doseMg,
+    required InjectionSite site,
+    DateTime? administeredAt,
+    String? notes,
+  }) async {
+    final administered = administeredAt ?? DateTime.now();
+    final entry = DoseEntry(
+      compoundName: compoundName.trim(),
+      doseMg: doseMg,
+      administeredAt: administered,
+      injectionSite: site,
+      notes: notes,
+    );
+
+    try {
+      await _repository.saveDoseEntry(entry);
+    } catch (error, stackTrace) {
+      if (mounted) {
+        state = state.copyWith(isLoading: false, error: error);
+      }
+      debugPrintStack(stackTrace: stackTrace, label: 'saveDoseEntry failed');
+      return false;
+    }
+
+    await _scheduleNextReminder(entry);
+    return true;
+  }
+
+  /// Deletes an injection by database [id].
+  Future<bool> deleteInjection(int id) async {
+    try {
+      await _repository.deleteDoseEntry(id);
+      return true;
+    } catch (error, stackTrace) {
+      if (mounted) {
+        state = state.copyWith(isLoading: false, error: error);
+      }
+      debugPrintStack(stackTrace: stackTrace, label: 'deleteDoseEntry failed');
+      return false;
+    }
+  }
+
+  void _subscribe() {
+    _subscription?.cancel();
+    _subscription = _repository.watchDoseEntries().listen(
+      (entries) => _reduce(entries, DateTime.now()),
+      onError: (Object error, StackTrace stackTrace) {
+        debugPrintStack(
+          stackTrace: stackTrace,
+          label: 'watchDoseEntries failed',
+        );
+        if (mounted) {
+          state = state.copyWith(isLoading: false, error: error);
+        }
+      },
+    );
+  }
+
+  /// Folds a new history snapshot into the derived presentation state.
+  void _reduce(List<DoseEntry> entries, DateTime at) {
+    state = MedicationState(
+      entries: entries,
+      isLoading: false,
+      error: null,
+      evaluatedAt: at,
+      serumLevelMg: _calculator.evaluate(doses: entries, at: at),
+      suggestedSite: _suggestSite(entries),
+    );
+  }
+
+  /// Rotation policy wrapper: builds the per-quadrant "last used" map from
+  /// the history and delegates to the domain's least-recently-used rule.
+  InjectionSite _suggestSite(List<DoseEntry> entries) {
+    final lastUsed = <InjectionSite, DateTime>{};
+    for (final entry in entries) {
+      final site = entry.injectionSite;
+      if (site == null) continue;
+      final previous = lastUsed[site];
+      if (previous == null || entry.administeredAt.isAfter(previous)) {
+        lastUsed[site] = entry.administeredAt;
+      }
+    }
+    return InjectionSite.leastRecentlyUsed(lastUsed);
+  }
+
+  /// Schedules the next dose reminder one dosing interval after the entry.
+  Future<void> _scheduleNextReminder(DoseEntry entry) async {
+    final compound = Glp1Compound.tryParse(entry.compoundName);
+    final interval = compound?.dosingInterval ?? const Duration(hours: 24);
+    final displayName = compound?.displayName ?? entry.compoundName;
+
+    try {
+      await _notifications.requestPermissions();
+      await _notifications.scheduleDoseReminder(
+        scheduledAt: entry.administeredAt.add(interval),
+        title: '$displayName dose due',
+        body: 'Time for your next $displayName injection. '
+            'Rotate to a new site: ${_suggestSite(state.entries).displayName}.',
+        payload: 'medication:dose_reminder',
+      );
+    } catch (error, stackTrace) {
+      // Reminder scheduling is best-effort: the injection itself already
+      // persisted, so a missing notification must not surface as an error.
+      debugPrintStack(
+        stackTrace: stackTrace,
+        label: 'scheduleDoseReminder failed (non-fatal)',
+      );
+    }
+  }
+
+  @override
+  void dispose() {
+    _subscription?.cancel();
+    super.dispose();
+  }
+}
+
+/// Exposes the [MedicationController] to the widget tree.
+final medicationControllerProvider =
+    StateNotifierProvider<MedicationController, MedicationState>(
+  (ref) => MedicationController(
+    ref.watch(medicationRepositoryProvider),
+    ref.watch(notificationServiceProvider),
+  ),
+);
\ No newline at end of file
diff --git a/lib/features/medication/presentation/screens/medication_screen.dart b/lib/features/medication/presentation/screens/medication_screen.dart
index a31eec1..68cc1f5 100644
--- a/lib/features/medication/presentation/screens/medication_screen.dart
+++ b/lib/features/medication/presentation/screens/medication_screen.dart
@@ -1,50 +1,366 @@
-import 'package:flutter/material.dart';
+import 'dart:async';
 
-class MedicationScreen extends StatelessWidget {
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import '../../../../core/utils/date_time_utils.dart';
+import '../../domain/entities/dose_entry.dart';
+import '../../domain/entities/injection_site.dart';
+import '../../domain/value_objects/glp1_compound.dart';
+import '../controllers/medication_controller.dart';
+import '../widgets/log_injection_dialog.dart';
+
+/// GLP-1 tracking dashboard.
+///
+/// Renders the pharmacokinetic serum estimate, the least-recently-used
+/// injection-site suggestion, an interactive logging flow, and the persisted
+/// injection history. A periodic ticker keeps the decaying serum value fresh
+/// while the screen is visible.
+class MedicationScreen extends ConsumerStatefulWidget {
   const MedicationScreen({super.key});
 
+  @override
+  ConsumerState<MedicationScreen> createState() => _MedicationScreenState();
+}
+
+class _MedicationScreenState extends ConsumerState<MedicationScreen> {
+  Timer? _serumTicker;
+
+  @override
+  void initState() {
+    super.initState();
+    // Serum concentration decays continuously; refresh the estimate once a
+    // minute so the displayed mg value tracks the pharmacokinetic model.
+    _serumTicker = Timer.periodic(
+      const Duration(minutes: 1),
+      (_) => ref.read(medicationControllerProvider.notifier).refreshSerumLevel(),
+    );
+  }
+
+  @override
+  void dispose() {
+    _serumTicker?.cancel();
+    super.dispose();
+  }
+
   @override
   Widget build(BuildContext context) {
+    final state = ref.watch(medicationControllerProvider);
+
     return Scaffold(
-      appBar: AppBar(title: const Text('Medication Schedule')),
-      body: ListView(
-        padding: const EdgeInsets.all(16.0),
-        children: [
-          Card(
+      appBar: AppBar(title: const Text('Medication Tracker')),
+      body: Center(
+        child: ConstrainedBox(
+          constraints: const BoxConstraints(maxWidth: 720),
+          child: _buildBody(context, state),
+        ),
+      ),
+    );
+  }
+
+  Widget _buildBody(BuildContext context, MedicationState state) {
+    if (state.isLoading && state.entries.isEmpty) {
+      return const Center(child: CircularProgressIndicator());
+    }
+
+    if (state.error != null && state.entries.isEmpty) {
+      return _ErrorView(
+        error: state.error!,
+        onRetry: () =>
+            ref.read(medicationControllerProvider.notifier).retry(),
+      );
+    }
+
+    return ListView(
+      padding: const EdgeInsets.all(16),
+      children: [
+        _SerumLevelCard(state: state),
+        const SizedBox(height: 16),
+        _SiteRotationCard(state: state),
+        const SizedBox(height: 16),
+        if (state.error != null) ...[
+          _InlineErrorBanner(error: state.error!),
+          const SizedBox(height: 16),
+        ],
+        FilledButton.icon(
+          onPressed: () => _logInjection(state),
+          icon: const Icon(Icons.add_circle_outline),
+          label: const Text('Log Injection'),
+        ),
+        const SizedBox(height: 24),
+        if (state.entries.isNotEmpty) ...[
+          Text(
+            'Recent Injections',
+            style: Theme.of(context)
+                .textTheme
+                .titleMedium
+                ?.copyWith(fontWeight: FontWeight.bold),
+          ),
+          const SizedBox(height: 8),
+          for (final entry in state.entries.take(20)) _HistoryTile(entry: entry),
+        ] else
+          const Card(
             child: Padding(
-              padding: const EdgeInsets.all(16.0),
-              child: Column(
-                crossAxisAlignment: CrossAxisAlignment.start,
-                children: [
-                  const Text(
-                    'Next Scheduled Dose',
-                    style: TextStyle(fontWeight: FontWeight.bold, fontSize: 16),
-                  ),
-                  const SizedBox(height: 8),
-                  const Text('Tirzepatide - 5.0 mg'),
-                  const Text('Scheduled: Monday at 08:00 AM'),
-                  const SizedBox(height: 16),
-                  FilledButton.icon(
-                    onPressed: () {},
-                    icon: const Icon(Icons.check_circle_outline),
-                    label: const Text('Log Injection Now'),
-                  ),
-                ],
+              padding: EdgeInsets.all(16),
+              child: Text(
+                'No injections logged yet. Tap "Log Injection" to start '
+                'tracking your doses and site rotation.',
               ),
             ),
           ),
-          const SizedBox(height: 16),
-          const Text(
-            'Recommended Injection Site Rotation',
-            style: TextStyle(fontWeight: FontWeight.bold, fontSize: 16),
+      ],
+    );
+  }
+
+  /// Opens the logging dialog pre-filled with the rotation suggestion and the
+  /// latest compound, then surfaces the outcome.
+  Future<void> _logInjection(MedicationState state) async {
+    final result = await showLogInjectionDialog(
+      context,
+      initialCompound: Glp1Compound.tryParse(state.latestDose?.compoundName),
+      initialSite: state.suggestedSite,
+    );
+    if (!mounted || result == null) return;
+
+    ScaffoldMessenger.of(context)
+      ..hideCurrentSnackBar()
+      ..showSnackBar(
+        SnackBar(
+          content: Text(
+            result
+                ? 'Injection logged. Next reminder scheduled for your next dose.'
+                : 'Could not log the injection. Please try again.',
           ),
-          const SizedBox(height: 8),
-          const ListTile(
-            leading: Icon(Icons.my_location, color: Colors.teal),
-            title: Text('Left Lower Abdomen'),
-            subtitle: Text('Last used 28 days ago'),
-          ),
-        ],
+          backgroundColor: result ? null : Theme.of(context).colorScheme.error,
+        ),
+      );
+  }
+}
+
+/// Displays the current estimated serum concentration with the active
+/// compound's half-life context and next dose due time.
+class _SerumLevelCard extends StatelessWidget {
+  const _SerumLevelCard({required this.state});
+
+  final MedicationState state;
+
+  @override
+  Widget build(BuildContext context) {
+    final latest = state.latestDose;
+    final compound = Glp1Compound.tryParse(latest?.compoundName);
+
+    return Card(
+      child: Padding(
+        padding: const EdgeInsets.all(16),
+        child: Column(
+          crossAxisAlignment: CrossAxisAlignment.start,
+          children: [
+            Text(
+              'Estimated Active Serum Level',
+              style: Theme.of(context)
+                  .textTheme
+                  .titleMedium
+                  ?.copyWith(fontWeight: FontWeight.bold),
+            ),
+            const SizedBox(height: 8),
+            Text(
+              '${state.serumLevelMg.toStringAsFixed(3)} mg',
+              style: Theme.of(context).textTheme.headlineMedium,
+            ),
+            const SizedBox(height: 4),
+            if (compound != null)
+              Text(
+                '${compound.displayName} · half-life '
+                '${compound.halfLifeHours.toStringAsFixed(0)} h',
+              )
+            else if (latest != null)
+              Text(latest.compoundName)
+            else
+              const Text('No injections logged yet.'),
+            Text('Evaluated ${formatDateTime(state.evaluatedAt)}'),
+            if (state.nextDoseDueAt != null) ...[
+              const SizedBox(height: 8),
+              Text(
+                'Next dose due ${formatDateTime(state.nextDoseDueAt!)}',
+                style: const TextStyle(fontWeight: FontWeight.w600),
+              ),
+            ],
+          ],
+        ),
+      ),
+    );
+  }
+}
+
+/// Shows the least-recently-used quadrant recommended for the next
+/// injection, including how long ago that site was last used.
+class _SiteRotationCard extends StatelessWidget {
+  const _SiteRotationCard({required this.state});
+
+  final MedicationState state;
+
+  @override
+  Widget build(BuildContext context) {
+    final site = state.suggestedSite;
+    final lastUsed = state.lastUsedAt(site);
+
+    return Card(
+      child: Padding(
+        padding: const EdgeInsets.all(16),
+        child: Column(
+          crossAxisAlignment: CrossAxisAlignment.start,
+          children: [
+            Text(
+              'Suggested Next Injection Site',
+              style: Theme.of(context)
+                  .textTheme
+                  .titleMedium
+                  ?.copyWith(fontWeight: FontWeight.bold),
+            ),
+            const SizedBox(height: 8),
+            ListTile(
+              contentPadding: EdgeInsets.zero,
+              leading: const Icon(Icons.my_location, color: Colors.teal),
+              title: Text(site.displayName),
+              subtitle: Text(
+                '${site.region.displayName} · '
+                '${lastUsed == null ? 'never used' : 'last used ${formatRelativeTime(lastUsed)}'}',
+              ),
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}
+
+/// A single persisted injection row with an optional delete action.
+class _HistoryTile extends ConsumerWidget {
+  const _HistoryTile({required this.entry});
+
+  final DoseEntry entry;
+
+  @override
+  Widget build(BuildContext context, WidgetRef ref) {
+    return ListTile(
+      leading: Icon(_regionIcon(entry.injectionSite?.region)),
+      title: Text(
+        '${entry.compoundName} · ${entry.doseMg.toStringAsFixed(2)} mg',
+      ),
+      subtitle: Text(
+        '${formatDateTime(entry.administeredAt)} · '
+        '${entry.injectionSite?.displayName ?? 'Site not recorded'}',
+      ),
+      trailing: entry.id == null
+          ? null
+          : PopupMenuButton<String>(
+              onSelected: (value) {
+                if (value == 'delete') _confirmDelete(context, ref);
+              },
+              itemBuilder: (context) => const [
+                PopupMenuItem(value: 'delete', child: Text('Delete')),
+              ],
+            ),
+    );
+  }
+
+  Future<void> _confirmDelete(BuildContext context, WidgetRef ref) async {
+    final confirmed = await showDialog<bool>(
+      context: context,
+      builder: (context) => AlertDialog(
+        title: const Text('Delete injection?'),
+        content: Text(
+          '${entry.compoundName} · ${formatDateTime(entry.administeredAt)}',
+        ),
+        actions: [
+          TextButton(
+            onPressed: () => Navigator.of(context).pop(false),
+            child: const Text('Cancel'),
+          ),
+          FilledButton(
+            onPressed: () => Navigator.of(context).pop(true),
+            child: const Text('Delete'),
+          ),
+        ],
+      ),
+    );
+    if (confirmed != true) return;
+
+    await ref
+        .read(medicationControllerProvider.notifier)
+        .deleteInjection(entry.id!);
+  }
+
+  IconData _regionIcon(InjectionBodyRegion? region) {
+    switch (region) {
+      case InjectionBodyRegion.abdomen:
+        return Icons.album;
+      case InjectionBodyRegion.thigh:
+        return Icons.directions_walk;
+      case InjectionBodyRegion.upperArm:
+        return Icons.accessibility_new;
+      case null:
+        return Icons.help_outline;
+    }
+  }
+}
+
+/// Full-screen failure state shown when no history could be loaded at all.
+class _ErrorView extends StatelessWidget {
+  const _ErrorView({required this.error, required this.onRetry});
+
+  final Object error;
+  final VoidCallback onRetry;
+
+  @override
+  Widget build(BuildContext context) {
+    return Center(
+      child: Padding(
+        padding: const EdgeInsets.all(24),
+        child: Column(
+          mainAxisSize: MainAxisSize.min,
+          children: [
+            const Icon(Icons.error_outline, size: 48, color: Colors.red),
+            const SizedBox(height: 12),
+            Text(
+              'Could not load your injection history.',
+              style: Theme.of(context).textTheme.titleMedium,
+              textAlign: TextAlign.center,
+            ),
+            const SizedBox(height: 4),
+            Text('$error', textAlign: TextAlign.center),
+            const SizedBox(height: 16),
+            FilledButton.icon(
+              onPressed: onRetry,
+              icon: const Icon(Icons.refresh),
+              label: const Text('Retry'),
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}
+
+/// Non-blocking warning shown when a write failed while history is visible.
+class _InlineErrorBanner extends StatelessWidget {
+  const _InlineErrorBanner({required this.error});
+
+  final Object error;
+
+  @override
+  Widget build(BuildContext context) {
+    return Card(
+      color: Theme.of(context).colorScheme.errorContainer,
+      child: Padding(
+        padding: const EdgeInsets.all(12),
+        child: Row(
+          children: [
+            const Icon(Icons.warning_amber_rounded),
+            const SizedBox(width: 8),
+            Expanded(child: Text('$error')),
+          ],
+        ),
       ),
     );
   }
diff --git a/lib/features/medication/presentation/widgets/log_injection_dialog.dart b/lib/features/medication/presentation/widgets/log_injection_dialog.dart
new file mode 100644
index 0000000..d867c3a
--- /dev/null
+++ b/lib/features/medication/presentation/widgets/log_injection_dialog.dart
@@ -0,0 +1,231 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import '../../../../core/utils/date_time_utils.dart';
+import '../../domain/entities/injection_site.dart';
+import '../../domain/value_objects/glp1_compound.dart';
+import '../controllers/medication_controller.dart';
+
+/// Opens the interactive injection logging dialog.
+///
+/// Resolves to `true` when the injection was persisted, `false` when saving
+/// failed, and `null` when the user dismissed the dialog.
+Future<bool?> showLogInjectionDialog(
+  BuildContext context, {
+  Glp1Compound? initialCompound,
+  InjectionSite? initialSite,
+}) {
+  return showDialog<bool>(
+    context: context,
+    builder: (context) => LogInjectionDialog(
+      initialCompound: initialCompound,
+      initialSite: initialSite,
+    ),
+  );
+}
+
+/// Form for logging a GLP-1 injection: compound, dosage, administration
+/// timestamp, and the 12-quadrant rotation site selector.
+class LogInjectionDialog extends ConsumerStatefulWidget {
+  const LogInjectionDialog({
+    super.key,
+    this.initialCompound,
+    this.initialSite,
+  });
+
+  /// Compound pre-selected from the latest logged dose.
+  final Glp1Compound? initialCompound;
+
+  /// Quadrant pre-selected from the rotation policy's suggestion.
+  final InjectionSite? initialSite;
+
+  @override
+  ConsumerState<LogInjectionDialog> createState() => _LogInjectionDialogState();
+}
+
+class _LogInjectionDialogState extends ConsumerState<LogInjectionDialog> {
+  late Glp1Compound _compound =
+      widget.initialCompound ?? Glp1Compound.semaglutide;
+  late InjectionSite _site =
+      widget.initialSite ?? InjectionSite.abdomenRightUpper;
+  late DateTime _administeredAt = DateTime.now();
+
+  late final TextEditingController _doseController = TextEditingController(
+    text: (widget.initialCompound ?? Glp1Compound.semaglutide)
+        .defaultDoseMg
+        .toString(),
+  );
+  final GlobalKey<FormState> _formKey = GlobalKey<FormState>();
+  bool _isSaving = false;
+
+  @override
+  Widget build(BuildContext context) {
+    return AlertDialog(
+      title: const Text('Log Injection'),
+      content: SizedBox(
+        width: 440,
+        child: Form(
+          key: _formKey,
+          child: SingleChildScrollView(
+            child: Column(
+              mainAxisSize: MainAxisSize.min,
+              crossAxisAlignment: CrossAxisAlignment.stretch,
+              children: [
+                DropdownButtonFormField<Glp1Compound>(
+                  initialValue: _compound,
+                  decoration: const InputDecoration(
+                    labelText: 'Compound',
+                    border: OutlineInputBorder(),
+                  ),
+                  items: [
+                    for (final compound in Glp1Compound.values)
+                      DropdownMenuItem(
+                        value: compound,
+                        child: Text(
+                          '${compound.displayName} (${compound.brandNames})',
+                        ),
+                      ),
+                  ],
+                  onChanged: (compound) {
+                    if (compound == null) return;
+                    setState(() {
+                      _compound = compound;
+                      _doseController.text = compound.defaultDoseMg.toString();
+                    });
+                  },
+                ),
+                const SizedBox(height: 16),
+                TextFormField(
+                  controller: _doseController,
+                  keyboardType:
+                      const TextInputType.numberWithOptions(decimal: true),
+                  decoration: const InputDecoration(
+                    labelText: 'Dosage (mg)',
+                    border: OutlineInputBorder(),
+                    suffixText: 'mg',
+                  ),
+                  validator: (value) {
+                    final dose = double.tryParse(value?.trim() ?? '');
+                    if (dose == null || dose <= 0) {
+                      return 'Enter a positive dose in mg';
+                    }
+                    return null;
+                  },
+                ),
+                const SizedBox(height: 16),
+                ListTile(
+                  contentPadding: EdgeInsets.zero,
+                  leading: const Icon(Icons.schedule),
+                  title: const Text('Administered at'),
+                  subtitle: Text(formatDateTime(_administeredAt)),
+                  trailing: const Icon(Icons.edit_calendar_outlined),
+                  onTap: _isSaving ? null : _pickAdministeredAt,
+                ),
+                const SizedBox(height: 8),
+                Text(
+                  'Injection site',
+                  style: Theme.of(context).textTheme.titleSmall,
+                ),
+                const SizedBox(height: 4),
+                for (final region in InjectionBodyRegion.values) ...[
+                  Padding(
+                    padding: const EdgeInsets.only(top: 8, bottom: 4),
+                    child: Text(
+                      region.displayName,
+                      style: Theme.of(context)
+                          .textTheme
+                          .labelLarge
+                          ?.copyWith(color: Colors.grey),
+                    ),
+                  ),
+                  Wrap(
+                    spacing: 8,
+                    runSpacing: 4,
+                    children: [
+                      for (final site in InjectionSite.values
+                          .where((site) => site.region == region))
+                        ChoiceChip(
+                          label: Text(site.shortLabel),
+                          selected: _site == site,
+                          onSelected: _isSaving
+                              ? null
+                              : (_) => setState(() => _site = site),
+                        ),
+                    ],
+                  ),
+                ],
+              ],
+            ),
+          ),
+        ),
+      ),
+      actions: [
+        TextButton(
+          onPressed: _isSaving ? null : () => Navigator.of(context).pop(),
+          child: const Text('Cancel'),
+        ),
+        FilledButton.icon(
+          onPressed: _isSaving ? null : _save,
+          icon: _isSaving
+              ? const SizedBox(
+                  width: 16,
+                  height: 16,
+                  child: CircularProgressIndicator(strokeWidth: 2),
+                )
+              : const Icon(Icons.check_circle_outline),
+          label: const Text('Log Injection'),
+        ),
+      ],
+    );
+  }
+
+  /// Lets the user backdate the injection via sequential date/time pickers.
+  Future<void> _pickAdministeredAt() async {
+    final now = DateTime.now();
+    final pickedDate = await showDatePicker(
+      context: context,
+      initialDate: _administeredAt,
+      firstDate: DateTime(now.year - 1),
+      lastDate: now,
+    );
+    if (pickedDate == null || !mounted) return;
+
+    final pickedTime = await showTimePicker(
+      context: context,
+      initialTime: TimeOfDay.fromDateTime(_administeredAt),
+    );
+    if (!mounted) return;
+
+    setState(() {
+      _administeredAt = DateTime(
+        pickedDate.year,
+        pickedDate.month,
+        pickedDate.day,
+        pickedTime?.hour ?? _administeredAt.hour,
+        pickedTime?.minute ?? _administeredAt.minute,
+      );
+    });
+  }
+
+  Future<void> _save() async {
+    if (!(_formKey.currentState?.validate() ?? false)) return;
+    final doseMg = double.parse(_doseController.text.trim());
+
+    setState(() => _isSaving = true);
+    final saved =
+        await ref.read(medicationControllerProvider.notifier).logInjection(
+              compoundName: _compound.displayName,
+              doseMg: doseMg,
+              site: _site,
+              administeredAt: _administeredAt,
+            );
+    if (!mounted) return;
+    Navigator.of(context).pop(saved);
+  }
+
+  @override
+  void dispose() {
+    _doseController.dispose();
+    super.dispose();
+  }
+}
\ No newline at end of file
diff --git a/pubspec.lock b/pubspec.lock
index f2aef03..2ae1e89 100644
--- a/pubspec.lock
+++ b/pubspec.lock
@@ -1059,7 +1059,7 @@ packages:
     source: hosted
     version: "0.7.12"
   timezone:
-    dependency: transitive
+    dependency: "direct main"
     description:
       name: timezone
       sha256: "2236ec079a174ce07434e89fcd3fcda430025eb7692244139a9cf54fdcf1fc7d"
diff --git a/pubspec.yaml b/pubspec.yaml
index 5b40c45..b2a9978 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -43,6 +43,8 @@ dependencies:
   path: ^1.9.1
   path_provider: ^2.1.6
   sqlite3: ^3.4.0
+  # Scheduling support for flutter_local_notifications (zonedSchedule).
+  timezone: ^0.9.4
 
 dev_dependencies:
   flutter_test:
diff --git a/test/features/auth/dev_auth_bypass_test.dart b/test/features/auth/dev_auth_bypass_test.dart
new file mode 100644
index 0000000..cd22003
--- /dev/null
+++ b/test/features/auth/dev_auth_bypass_test.dart
@@ -0,0 +1,107 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:glp1_companion/core/platform/notification_service.dart';
+import 'package:glp1_companion/features/auth/presentation/controllers/dev_auth_bypass.dart';
+import 'package:glp1_companion/features/auth/presentation/screens/login_screen.dart';
+import 'package:glp1_companion/features/medication/domain/entities/dose_entry.dart';
+import 'package:glp1_companion/features/medication/domain/repositories/medication_repository.dart';
+import 'package:glp1_companion/features/medication/presentation/controllers/medication_controller.dart';
+import 'package:glp1_companion/main.dart';
+
+/// Empty history so the medication shell renders without touching drift.
+class _EmptyMedicationRepository implements IMedicationRepository {
+  @override
+  Stream<List<DoseEntry>> watchDoseEntries({int limit = 500}) =>
+      Stream.value(const []);
+
+  @override
+  Future<List<DoseEntry>> getDoseEntries({int limit = 500}) async => const [];
+
+  @override
+  Future<DoseEntry> saveDoseEntry(DoseEntry entry) async => entry;
+
+  @override
+  Future<void> deleteDoseEntry(int id) async {}
+
+  @override
+  Future<DoseEntry?> getLatestAdministeredDose() async => null;
+}
+
+class _NoopNotificationService implements INotificationService {
+  @override
+  Future<void> initialize() async {}
+
+  @override
+  Future<bool> requestPermissions() async => false;
+
+  @override
+  Future<bool> scheduleDoseReminder({
+    required DateTime scheduledAt,
+    required String title,
+    required String body,
+    int id = kDoseReminderNotificationId,
+    String? payload,
+  }) async =>
+      false;
+
+  @override
+  Future<void> cancelReminder(int id) async {}
+
+  @override
+  Future<void> cancelAllReminders() async {}
+}
+
+Widget _buildApp() => ProviderScope(
+      overrides: [
+        medicationRepositoryProvider
+            .overrideWithValue(_EmptyMedicationRepository()),
+        notificationServiceProvider
+            .overrideWithValue(_NoopNotificationService()),
+      ],
+      child: const GLP1CompanionApp(),
+    );
+
+void main() {
+  testWidgets('unauthenticated sessions land on the login screen', (tester) async {
+    await tester.pumpWidget(_buildApp());
+    await tester.pumpAndSettle();
+
+    expect(find.byType(LoginScreen), findsOneWidget);
+  });
+
+  testWidgets('dev bypass button skips sign-in and opens the medication shell',
+      (tester) async {
+    // A tall phone-like viewport keeps the whole login column on-screen.
+    tester.view.physicalSize = const Size(1080, 2400);
+    tester.view.devicePixelRatio = 1.0;
+    addTearDown(tester.view.reset);
+
+    await tester.pumpWidget(_buildApp());
+    await tester.pumpAndSettle();
+
+    // Debug builds expose the bypass affordance on the login screen.
+    final bypassButton = find.text('Continue Without Signing In');
+    expect(bypassButton, findsOneWidget);
+
+    await tester.tap(bypassButton);
+    await tester.pumpAndSettle();
+
+    // The guard admits the session and routes into the protected shell.
+    expect(find.byType(LoginScreen), findsNothing);
+    expect(find.text('Medication Tracker'), findsOneWidget);
+    // The dev banner marks the session as not signed in.
+    expect(find.text('Development mode - not signed in'), findsOneWidget);
+  });
+
+  testWidgets('bypass resets to disabled on every cold start', (tester) async {
+    await tester.pumpWidget(
+      const ProviderScope(child: SizedBox()),
+    );
+
+    final container = ProviderScope.containerOf(
+      tester.element(find.byType(SizedBox)),
+    );
+    expect(container.read(devAuthBypassProvider), isFalse);
+  });
+}
\ No newline at end of file
diff --git a/test/features/medication/data/medication_repository_test.dart b/test/features/medication/data/medication_repository_test.dart
new file mode 100644
index 0000000..fd1e1e6
--- /dev/null
+++ b/test/features/medication/data/medication_repository_test.dart
@@ -0,0 +1,112 @@
+import 'package:drift/native.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:glp1_companion/core/database/app_database.dart';
+import 'package:glp1_companion/features/medication/data/repositories/medication_repository.dart';
+import 'package:glp1_companion/features/medication/domain/entities/dose_entry.dart';
+import 'package:glp1_companion/features/medication/domain/entities/injection_site.dart';
+
+void main() {
+  late AppDatabase database;
+  late MedicationRepository repository;
+
+  final administeredAt = DateTime(2026, 10, 16, 8);
+
+  setUp(() {
+    database = AppDatabase.forTesting(NativeDatabase.memory());
+    repository = MedicationRepository(database);
+  });
+
+  tearDown(() => database.close());
+
+  DoseEntry makeEntry({
+    int? id,
+    String compound = 'Tirzepatide',
+    double doseMg = 5,
+    DateTime? administered,
+    InjectionSite? site,
+  }) =>
+      DoseEntry(
+        id: id,
+        compoundName: compound,
+        doseMg: doseMg,
+        administeredAt: administered ?? administeredAt,
+        injectionSite: site,
+      );
+
+  group('MedicationRepository', () {
+    test('saveDoseEntry assigns an id and persists all fields', () async {
+      final saved = await repository.saveDoseEntry(
+        makeEntry(site: InjectionSite.abdomenLeftLower, doseMg: 4.5),
+      );
+
+      expect(saved.id, isNotNull);
+
+      final fetched = await repository.getDoseEntries();
+      expect(fetched, hasLength(1));
+      expect(fetched.single.id, saved.id);
+      expect(fetched.single.compoundName, 'Tirzepatide');
+      expect(fetched.single.doseMg, 4.5);
+      expect(fetched.single.administeredAt, administeredAt);
+      expect(fetched.single.injectionSite, InjectionSite.abdomenLeftLower);
+    });
+
+    test('getDoseEntries returns entries newest first', () async {
+      await repository.saveDoseEntry(
+        makeEntry(administered: administeredAt.subtract(const Duration(days: 7))),
+      );
+      await repository.saveDoseEntry(
+        makeEntry(administered: administeredAt),
+      );
+
+      final entries = await repository.getDoseEntries();
+      expect(entries, hasLength(2));
+      expect(entries.first.administeredAt, administeredAt);
+      expect(entries.last.administeredAt,
+          administeredAt.subtract(const Duration(days: 7)));
+    });
+
+    test('watchDoseEntries emits the current history on subscription',
+        () async {
+      await repository.saveDoseEntry(makeEntry());
+
+      final first = await repository.watchDoseEntries().first;
+      expect(first, hasLength(1));
+    });
+
+    test('saveDoseEntry updates an existing row when id is present', () async {
+      final saved = await repository.saveDoseEntry(makeEntry());
+      final updated = await repository.saveDoseEntry(
+        makeEntry(id: saved.id, doseMg: 9.5, site: InjectionSite.thighLeftAnterior),
+      );
+
+      expect(updated.id, saved.id);
+      final entries = await repository.getDoseEntries();
+      expect(entries, hasLength(1));
+      expect(entries.single.doseMg, 9.5);
+      expect(entries.single.injectionSite, InjectionSite.thighLeftAnterior);
+    });
+
+    test('deleteDoseEntry removes the row', () async {
+      final saved = await repository.saveDoseEntry(makeEntry());
+      await repository.deleteDoseEntry(saved.id!);
+
+      expect(await repository.getDoseEntries(), isEmpty);
+    });
+
+    test('getLatestAdministeredDose ignores an empty history', () async {
+      expect(await repository.getLatestAdministeredDose(), isNull);
+    });
+
+    test('getLatestAdministeredDose returns the most recent administration',
+        () async {
+      final older = await repository.saveDoseEntry(
+        makeEntry(administered: administeredAt.subtract(const Duration(days: 7))),
+      );
+      await repository.saveDoseEntry(makeEntry(administered: administeredAt));
+
+      final latest = await repository.getLatestAdministeredDose();
+      expect(latest?.id, isNot(older.id));
+      expect(latest?.administeredAt, administeredAt);
+    });
+  });
+}
\ No newline at end of file
diff --git a/test/features/medication/domain/calculate_serum_level_test.dart b/test/features/medication/domain/calculate_serum_level_test.dart
new file mode 100644
index 0000000..2634a2d
--- /dev/null
+++ b/test/features/medication/domain/calculate_serum_level_test.dart
@@ -0,0 +1,141 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:glp1_companion/features/medication/domain/entities/dose_entry.dart';
+import 'package:glp1_companion/features/medication/domain/usecases/calculate_serum_level.dart';
+
+DoseEntry _dose({
+  required String compound,
+  required double mg,
+  required DateTime administeredAt,
+}) =>
+    DoseEntry(
+      compoundName: compound,
+      doseMg: mg,
+      administeredAt: administeredAt,
+    );
+
+void main() {
+  const calculator = CalculateSerumLevel();
+  final now = DateTime(2026, 10, 16, 12);
+
+  group('CalculateSerumLevel', () {
+    test('empty history yields zero concentration', () {
+      expect(calculator.evaluate(doses: const [], at: now), 0);
+    });
+
+    test('a fresh dose contributes its full amount', () {
+      final doses = [
+        _dose(compound: 'Semaglutide', mg: 1, administeredAt: now),
+      ];
+      expect(calculator.evaluate(doses: doses, at: now), closeTo(1, 1e-9));
+    });
+
+    test('dose halves exactly after one half-life', () {
+      final doses = [
+        _dose(
+          compound: 'Semaglutide',
+          mg: 2,
+          administeredAt: now.subtract(const Duration(hours: 168)),
+        ),
+      ];
+      expect(calculator.evaluate(doses: doses, at: now), closeTo(1, 1e-9));
+    });
+
+    test('follows the published half-life per compound', () {
+      // Tirzepatide: 2.5 mg after 120 h (one half-life) -> 1.25 mg.
+      final tirzepatide = [
+        _dose(
+          compound: 'Tirzepatide',
+          mg: 2.5,
+          administeredAt: now.subtract(const Duration(hours: 120)),
+        ),
+      ];
+      expect(calculator.evaluate(doses: tirzepatide, at: now), closeTo(1.25, 1e-9));
+
+      // Liraglutide: 1.8 mg after 13 h (one half-life) -> 0.9 mg.
+      final liraglutide = [
+        _dose(
+          compound: 'Liraglutide',
+          mg: 1.8,
+          administeredAt: now.subtract(const Duration(hours: 13)),
+        ),
+      ];
+      expect(calculator.evaluate(doses: liraglutide, at: now), closeTo(0.9, 1e-9));
+    });
+
+    test('accumulates contributions across multiple doses', () {
+      final doses = [
+        _dose(
+          compound: 'Semaglutide',
+          mg: 1,
+          administeredAt: now.subtract(const Duration(hours: 168)),
+        ),
+        _dose(
+          compound: 'Semaglutide',
+          mg: 1,
+          administeredAt: now.subtract(const Duration(hours: 336)),
+        ),
+      ];
+      // 1 * 0.5 + 1 * 0.25 = 0.75 mg.
+      expect(calculator.evaluate(doses: doses, at: now), closeTo(0.75, 1e-9));
+    });
+
+    test('ignores doses timestamped in the future', () {
+      final doses = [
+        _dose(compound: 'Semaglutide', mg: 1, administeredAt: now),
+        _dose(
+          compound: 'Semaglutide',
+          mg: 5,
+          administeredAt: now.add(const Duration(hours: 24)),
+        ),
+      ];
+      expect(calculator.evaluate(doses: doses, at: now), closeTo(1, 1e-9));
+    });
+
+    test('unknown compounds fall back to the default half-life', () {
+      final doses = [
+        _dose(
+          compound: 'Compounded GLP-1',
+          mg: 4,
+          administeredAt: now.subtract(const Duration(hours: 168)),
+        ),
+      ];
+      expect(calculator.evaluate(doses: doses, at: now), closeTo(2, 1e-9));
+    });
+
+    test('brand names resolve to their compound half-life', () {
+      // "Ozempic" is semaglutide (168 h): 4 mg after 168 h -> 2 mg.
+      final doses = [
+        _dose(
+          compound: 'Ozempic',
+          mg: 4,
+          administeredAt: now.subtract(const Duration(hours: 168)),
+        ),
+      ];
+      expect(calculator.evaluate(doses: doses, at: now), closeTo(2, 1e-9));
+    });
+
+    test('call() mirrors evaluate()', () {
+      final doses = [
+        _dose(compound: 'Tirzepatide', mg: 3, administeredAt: now),
+      ];
+      expect(
+        calculator(doses: doses, at: now),
+        calculator.evaluate(doses: doses, at: now),
+      );
+    });
+
+    test('halfLifeHours override forces a single half-life', () {
+      final doses = [
+        _dose(compound: 'Tirzepatide', mg: 2, administeredAt: now),
+      ];
+      expect(
+        calculator.evaluate(
+          doses: doses,
+          at: now.add(const Duration(hours: 20)),
+          halfLifeHours: 20,
+        ),
+        closeTo(1, 1e-9),
+      );
+    });
+  });
+}
\ No newline at end of file
diff --git a/test/features/medication/domain/injection_site_test.dart b/test/features/medication/domain/injection_site_test.dart
new file mode 100644
index 0000000..b352e14
--- /dev/null
+++ b/test/features/medication/domain/injection_site_test.dart
@@ -0,0 +1,90 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:glp1_companion/features/medication/domain/entities/injection_site.dart';
+import 'package:glp1_companion/features/medication/domain/value_objects/glp1_compound.dart';
+
+void main() {
+  group('InjectionSite model', () {
+    test('exposes exactly twelve anatomical quadrants', () {
+      expect(InjectionSite.values, hasLength(12));
+    });
+
+    test('quadrants span three regions with four sites each', () {
+      for (final region in InjectionBodyRegion.values) {
+        expect(
+          InjectionSite.values.where((site) => site.region == region),
+          hasLength(4),
+          reason: 'region ${region.name} should own four quadrants',
+        );
+      }
+    });
+
+    test('tryParse round-trips the persistence key', () {
+      for (final site in InjectionSite.values) {
+        expect(InjectionSite.tryParse(site.persistenceKey), site);
+        expect(InjectionSite.tryParse(site.displayName), site);
+      }
+      expect(InjectionSite.tryParse(null), isNull);
+      expect(InjectionSite.tryParse('  '), isNull);
+      expect(InjectionSite.tryParse('unknown-quadrant'), isNull);
+    });
+  });
+
+  group('InjectionSite.leastRecentlyUsed', () {
+    final now = DateTime(2026, 10, 16);
+
+    test('never-used quadrants win over used ones', () {
+      final lastUsed = {
+        InjectionSite.abdomenRightUpper: now,
+        InjectionSite.thighLeftLateral: now.subtract(const Duration(days: 90)),
+      };
+      final suggestion = InjectionSite.leastRecentlyUsed(lastUsed);
+      expect(
+        lastUsed.containsKey(suggestion),
+        isFalse,
+        reason: 'suggested site must be one that was never used',
+      );
+    });
+
+    test('with full history the oldest site is chosen', () {
+      final lastUsed = {
+        for (var i = 0; i < InjectionSite.values.length; i++)
+          InjectionSite.values[i]: now.subtract(Duration(days: i)),
+      };
+      expect(
+        InjectionSite.leastRecentlyUsed(lastUsed),
+        InjectionSite.values.last,
+      );
+    });
+
+    test('ties resolve deterministically to declaration order', () {
+      final lastUsed = {
+        for (final site in InjectionSite.values) site: now,
+      };
+      expect(InjectionSite.leastRecentlyUsed(lastUsed), InjectionSite.values.first);
+
+      expect(InjectionSite.leastRecentlyUsed(const {}), InjectionSite.values.first);
+    });
+  });
+
+  group('Glp1Compound', () {
+    test('models the three supported agonists with published half-lives', () {
+      expect(Glp1Compound.semaglutide.halfLifeHours, 168);
+      expect(Glp1Compound.tirzepatide.halfLifeHours, 120);
+      expect(Glp1Compound.liraglutide.halfLifeHours, 13);
+    });
+
+    test('tryParse resolves generic names, brands, and free text', () {
+      expect(Glp1Compound.tryParse('semaglutide'), Glp1Compound.semaglutide);
+      expect(Glp1Compound.tryParse('OZEMPIC'), Glp1Compound.semaglutide);
+      expect(Glp1Compound.tryParse('Mounjaro'), Glp1Compound.tirzepatide);
+      expect(Glp1Compound.tryParse('Zepbound'), Glp1Compound.tirzepatide);
+      expect(Glp1Compound.tryParse('Saxenda'), Glp1Compound.liraglutide);
+      expect(
+        Glp1Compound.tryParse('Semaglutide (compounded)'),
+        Glp1Compound.semaglutide,
+      );
+      expect(Glp1Compound.tryParse('unknown'), isNull);
+      expect(Glp1Compound.tryParse(null), isNull);
+    });
+  });
+}
\ No newline at end of file
diff --git a/test/features/medication/presentation/medication_controller_test.dart b/test/features/medication/presentation/medication_controller_test.dart
new file mode 100644
index 0000000..aecbf1c
--- /dev/null
+++ b/test/features/medication/presentation/medication_controller_test.dart
@@ -0,0 +1,277 @@
+import 'dart:async';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:glp1_companion/core/platform/notification_service.dart';
+import 'package:glp1_companion/features/medication/domain/entities/dose_entry.dart';
+import 'package:glp1_companion/features/medication/domain/entities/injection_site.dart';
+import 'package:glp1_companion/features/medication/domain/repositories/medication_repository.dart';
+import 'package:glp1_companion/features/medication/domain/value_objects/glp1_compound.dart';
+import 'package:glp1_companion/features/medication/presentation/controllers/medication_controller.dart';
+
+/// In-memory [IMedicationRepository] that broadcasts every mutation.
+class _FakeMedicationRepository implements IMedicationRepository {
+  final List<DoseEntry> entries = [];
+  final StreamController<List<DoseEntry>> _changes =
+      StreamController<List<DoseEntry>>.broadcast(sync: true);
+  int _nextId = 1;
+  bool failNextSave = false;
+
+  void seed(List<DoseEntry> seeded) {
+    entries.addAll(seeded);
+    _emit();
+  }
+
+  void _emit() => _changes.add(List.unmodifiable(entries));
+
+  @override
+  Stream<List<DoseEntry>> watchDoseEntries({int limit = 500}) => _changes.stream;
+
+  @override
+  Future<List<DoseEntry>> getDoseEntries({int limit = 500}) async =>
+      List.unmodifiable(entries);
+
+  @override
+  Future<DoseEntry> saveDoseEntry(DoseEntry entry) async {
+    if (failNextSave) {
+      failNextSave = false;
+      throw StateError('write failed');
+    }
+    final stored = DoseEntry(
+      id: entry.id ?? _nextId++,
+      compoundName: entry.compoundName,
+      doseMg: entry.doseMg,
+      administeredAt: entry.administeredAt,
+      scheduledAt: entry.scheduledAt,
+      injectionSite: entry.injectionSite,
+      notes: entry.notes,
+    );
+    final index = entries.indexWhere((e) => e.id == stored.id);
+    if (index == -1) {
+      entries.insert(0, stored);
+    } else {
+      entries[index] = stored;
+    }
+    _emit();
+    return stored;
+  }
+
+  @override
+  Future<void> deleteDoseEntry(int id) async {
+    entries.removeWhere((entry) => entry.id == id);
+    _emit();
+  }
+
+  @override
+  Future<DoseEntry?> getLatestAdministeredDose() async =>
+      entries.isEmpty ? null : entries.first;
+
+  Future<void> close() => _changes.close();
+}
+
+/// Records scheduling calls instead of touching platform channels.
+class _RecordingNotificationService implements INotificationService {
+  final List<DateTime> scheduledTimes = [];
+  int permissionRequests = 0;
+
+  @override
+  Future<void> initialize() async {}
+
+  @override
+  Future<bool> requestPermissions() async {
+    permissionRequests++;
+    return true;
+  }
+
+  @override
+  Future<bool> scheduleDoseReminder({
+    required DateTime scheduledAt,
+    required String title,
+    required String body,
+    int id = kDoseReminderNotificationId,
+    String? payload,
+  }) async {
+    scheduledTimes.add(scheduledAt);
+    return true;
+  }
+
+  @override
+  Future<void> cancelReminder(int id) async {}
+
+  @override
+  Future<void> cancelAllReminders() async {}
+}
+
+DoseEntry _dose({
+  int? id,
+  String compound = 'Semaglutide',
+  double doseMg = 1,
+  required DateTime administeredAt,
+  InjectionSite? site,
+}) =>
+    DoseEntry(
+      id: id,
+      compoundName: compound,
+      doseMg: doseMg,
+      administeredAt: administeredAt,
+      injectionSite: site,
+    );
+
+void main() {
+  late _FakeMedicationRepository repository;
+  late _RecordingNotificationService notifications;
+
+  setUp(() {
+    repository = _FakeMedicationRepository();
+    notifications = _RecordingNotificationService();
+  });
+
+  tearDown(() => repository.close());
+
+  MedicationController buildController() {
+    final controller = MedicationController(repository, notifications);
+    addTearDown(controller.dispose);
+    return controller;
+  }
+
+  group('MedicationController', () {
+    test('starts in a loading state before history arrives', () {
+      final controller = buildController();
+      expect(controller.state.isLoading, isTrue);
+      expect(controller.state.entries, isEmpty);
+      expect(controller.state.suggestedSite, InjectionSite.values.first);
+    });
+
+    test('loads history, computes serum level, and suggests a rotation site',
+        () async {
+      final controller = buildController();
+      final oneHalfLifeAgo =
+          DateTime.now().subtract(Glp1Compound.semaglutide.dosingInterval);
+
+      repository.seed([
+        _dose(
+          administeredAt: oneHalfLifeAgo,
+          site: InjectionSite.abdomenRightUpper,
+        ),
+      ]);
+
+      expect(controller.state.isLoading, isFalse);
+      expect(controller.state.entries, hasLength(1));
+      // 1 mg after one semaglutide half-life decays to ~0.5 mg.
+      expect(controller.state.serumLevelMg, closeTo(0.5, 0.01));
+      expect(
+        controller.state.suggestedSite,
+        isNot(InjectionSite.abdomenRightUpper),
+      );
+      expect(
+        controller.state.suggestedSite,
+        InjectionSite.leastRecentlyUsed({
+          InjectionSite.abdomenRightUpper: oneHalfLifeAgo,
+        }),
+      );
+    });
+
+    test('nextDoseDueAt follows the compound dosing interval', () async {
+      final controller = buildController();
+      final administeredAt = DateTime.now().subtract(const Duration(days: 1));
+
+      repository.seed([
+        _dose(
+          compound: 'Liraglutide',
+          administeredAt: administeredAt,
+          site: InjectionSite.thighRightLateral,
+        ),
+      ]);
+
+      expect(
+        controller.state.nextDoseDueAt,
+        administeredAt.add(Glp1Compound.liraglutide.dosingInterval),
+      );
+    });
+
+    test('logInjection persists the entry and schedules the next reminder',
+        () async {
+      final controller = buildController();
+      final administeredAt = DateTime(2026, 10, 16, 8);
+
+      final saved = await controller.logInjection(
+        compoundName: 'Tirzepatide',
+        doseMg: 2.5,
+        site: InjectionSite.thighRightAnterior,
+        administeredAt: administeredAt,
+      );
+
+      expect(saved, isTrue);
+      expect(repository.entries, hasLength(1));
+      expect(
+        repository.entries.single.injectionSite,
+        InjectionSite.thighRightAnterior,
+      );
+      expect(notifications.permissionRequests, 1);
+      expect(notifications.scheduledTimes, [
+        administeredAt.add(Glp1Compound.tirzepatide.dosingInterval),
+      ]);
+    });
+
+    test('rotation moves away from the most recently used quadrant', () async {
+      final controller = buildController();
+
+      await controller.logInjection(
+        compoundName: 'Semaglutide',
+        doseMg: 0.25,
+        site: InjectionSite.abdomenRightUpper,
+      );
+
+      expect(controller.state.suggestedSite, InjectionSite.abdomenRightLower);
+    });
+
+    test('logInjection failure returns false and records the error', () async {
+      final controller = buildController();
+      repository.failNextSave = true;
+
+      final saved = await controller.logInjection(
+        compoundName: 'Semaglutide',
+        doseMg: 0.25,
+        site: InjectionSite.armLeftTricep,
+      );
+
+      expect(saved, isFalse);
+      expect(controller.state.error, isA<StateError>());
+      expect(notifications.scheduledTimes, isEmpty);
+    });
+
+    test('deleteInjection removes an entry from the stream', () async {
+      final controller = buildController();
+      final stored = await repository.saveDoseEntry(
+        _dose(
+          administeredAt: DateTime.now(),
+          site: InjectionSite.armRightLateral,
+        ),
+      );
+
+      expect(controller.state.entries, hasLength(1));
+      final removed = await controller.deleteInjection(stored.id!);
+
+      expect(removed, isTrue);
+      expect(controller.state.entries, isEmpty);
+    });
+
+    test('refreshSerumLevel re-evaluates decay at the requested time', () async {
+      final controller = buildController();
+      final administeredAt =
+          DateTime.now().subtract(const Duration(hours: 168));
+      repository.seed([
+        _dose(doseMg: 2, administeredAt: administeredAt),
+      ]);
+      final seededLevel = controller.state.serumLevelMg;
+
+      final later = administeredAt.add(const Duration(hours: 336));
+      controller.refreshSerumLevel(now: later);
+
+      // Two half-lives after administration: 2 mg -> 0.5 mg, below the
+      // freshly seeded estimate.
+      expect(controller.state.serumLevelMg, closeTo(0.5, 1e-9));
+      expect(controller.state.serumLevelMg, lessThan(seededLevel));
+      expect(controller.state.evaluatedAt, later);
+    });
+  });
+}
\ No newline at end of file