diff --git a/src/app/api/medications/[id]/inventory/[itemId]/route.ts b/src/app/api/medications/[id]/inventory/[itemId]/route.ts
index 1d02714a6..6fcc4895b 100644
--- a/src/app/api/medications/[id]/inventory/[itemId]/route.ts
+++ b/src/app/api/medications/[id]/inventory/[itemId]/route.ts
@@ -26,7 +26,10 @@ import {
   safeJson,
 } from "@/lib/api-response";
 import { updateInventoryItemSchema } from "@/lib/validations/medication";
-import { computeExpiresAt } from "@/lib/medications/inventory/state-machine";
+import {
+  computeExpiresAt,
+  computeInventoryState,
+} from "@/lib/medications/inventory/state-machine";
 
 type RouteParams = { params: Promise<{ id: string; itemId: string }> };
 
@@ -88,6 +91,27 @@ export const PATCH = apiHandler(
 
     const nextExpiresAt = computeExpiresAt(nextFirstUseAt, nextPrintedExpiry);
 
+    // Re-run the canonical state machine over the composed next-row
+    // view. The clause-by-clause updates above set the state ad-hoc
+    // (`ACTIVE → IN_USE` on first use, `* → USED_UP` on mark-as-used),
+    // but a back-dated `markAsFirstUseAt` whose 30-day window already
+    // elapsed should land at EXPIRED, not IN_USE. The state machine is
+    // pure and idempotent — running it once more with the composed view
+    // collapses every edge case onto the same decision tree the intake
+    // hook and the daily expire cron already share. USED_UP is terminal
+    // (dosesRemaining === 0 ⇒ USED_UP wins at clause 1), so the manual
+    // override remains sticky.
+    nextState = computeInventoryState(
+      {
+        state: nextState,
+        dosesTotal: existing.dosesTotal,
+        dosesRemaining: nextDosesRemaining,
+        firstUseAt: nextFirstUseAt,
+        printedExpiry: nextPrintedExpiry,
+      },
+      Date.now(),
+    );
+
     const updated = await prisma.medicationInventoryItem.update({
       where: { id: itemId },
       data: {
diff --git a/src/app/api/medications/[id]/inventory/__tests__/route.test.ts b/src/app/api/medications/[id]/inventory/__tests__/route.test.ts
index 6838b9f25..24a7a8d59 100644
--- a/src/app/api/medications/[id]/inventory/__tests__/route.test.ts
+++ b/src/app/api/medications/[id]/inventory/__tests__/route.test.ts
@@ -268,6 +268,42 @@ describe("PATCH /api/medications/[id]/inventory/[itemId]", () => {
     });
   });
 
+  it("flips ACTIVE → EXPIRED when markAsFirstUseAt is more than 30 days back-dated", async () => {
+    // Regression for the W19b state-machine bypass: composing the next
+    // state by hand sent ACTIVE → IN_USE on any first-use stamp, but a
+    // back-dated stamp whose 30-day window already lapsed must land at
+    // EXPIRED. Re-running `computeInventoryState` after the composed
+    // view closes the gap.
+    vi.mocked(getSession).mockResolvedValue(SESSION_OK as never);
+    vi.mocked(prisma.medicationInventoryItem.findUnique).mockResolvedValue(
+      existingActive as never,
+    );
+    vi.mocked(prisma.medicationInventoryItem.update).mockResolvedValue({
+      ...existingActive,
+      state: "EXPIRED",
+    } as never);
+
+    // 45 days in the past — well past the 30-day in-use window.
+    const backdated = new Date(
+      Date.now() - 45 * 24 * 60 * 60 * 1000,
+    ).toISOString();
+    const res = await PATCH(
+      jsonReq(
+        "http://localhost/api/medications/med-1/inventory/inv-1",
+        { markAsFirstUseAt: backdated },
+        "PATCH",
+      ),
+      { params: Promise.resolve({ id: "med-1", itemId: "inv-1" }) },
+    );
+    expect(res.status).toBe(200);
+    const updateCall = vi.mocked(prisma.medicationInventoryItem.update).mock
+      .calls[0][0];
+    expect(updateCall.data).toMatchObject({
+      state: "EXPIRED",
+      firstUseAt: new Date(backdated),
+    });
+  });
+
   it("rejects 404 when the item belongs to another user", async () => {
     vi.mocked(getSession).mockResolvedValue(SESSION_OK as never);
     vi.mocked(prisma.medicationInventoryItem.findUnique).mockResolvedValue({