diff --git a/database/migrations/2026_05_21_000002_lowercase_user_emails.php b/database/migrations/2026_05_21_000002_lowercase_user_emails.php
new file mode 100644
index 00000000..15f75848
--- /dev/null
+++ b/database/migrations/2026_05_21_000002_lowercase_user_emails.php
@@ -0,0 +1,64 @@
+<?php
+
+declare(strict_types=1);
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Support\Facades\DB;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     *
+     * @throws RuntimeException
+     */
+    public function up(): void
+    {
+        $duplicateEmails = DB::table('users')
+            ->selectRaw('LOWER(email) as normalized_email')
+            ->selectRaw('COUNT(*) as user_count')
+            ->selectRaw("STRING_AGG(id::text || ' <' || email || '>', ', ' ORDER BY email) as users")
+            ->where('is_placeholder', false)
+            ->groupByRaw('LOWER(email)')
+            ->havingRaw('COUNT(*) > 1')
+            ->orderBy('normalized_email')
+            ->get();
+
+        if ($duplicateEmails->isNotEmpty()) {
+            $duplicateEmailMessage = $duplicateEmails
+                ->take(20)
+                ->map(fn (\stdClass $duplicateEmail): string => sprintf(
+                    '%s (%d users: %s)',
+                    $duplicateEmail->normalized_email,
+                    $duplicateEmail->user_count,
+                    $duplicateEmail->users,
+                ))
+                ->implode('; ');
+
+            $remainingDuplicateCount = $duplicateEmails->count() - 20;
+            $remainingDuplicateMessage = $remainingDuplicateCount > 0
+                ? sprintf('; and %d more duplicate normalized emails', $remainingDuplicateCount)
+                : '';
+
+            throw new RuntimeException(
+                'Cannot lowercase users.email because doing so would create duplicate non-placeholder user emails and violate the unique index on users.email for non-placeholder users. Resolve these case-insensitive duplicates first: '.
+                $duplicateEmailMessage.
+                $remainingDuplicateMessage
+            );
+        }
+
+        DB::table('users')
+            ->whereRaw('email <> LOWER(email)')
+            ->update([
+                'email' => DB::raw('LOWER(email)'),
+            ]);
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        //
+    }
+};