diff --git a/src/lib/medications/inventory/__tests__/state-machine.test.ts b/src/lib/medications/inventory/__tests__/state-machine.test.ts
new file mode 100644
index 000000000..24fcc8bb9
--- /dev/null
+++ b/src/lib/medications/inventory/__tests__/state-machine.test.ts
@@ -0,0 +1,227 @@
+import { describe, expect, it } from "vitest";
+
+import {
+  computeExpiresAt,
+  computeInventoryState,
+  daysRemainingInUse,
+  decrementDose,
+  DEFAULT_IN_USE_WINDOW_DAYS,
+  type InventoryItemView,
+} from "../state-machine";
+
+const MS_PER_DAY = 24 * 60 * 60 * 1000;
+const NOW = new Date("2026-05-14T12:00:00Z");
+const NOW_MS = NOW.getTime();
+
+function makeItem(overrides: Partial<InventoryItemView> = {}): InventoryItemView {
+  return {
+    state: "ACTIVE",
+    dosesTotal: 4,
+    dosesRemaining: 4,
+    firstUseAt: null,
+    printedExpiry: null,
+    ...overrides,
+  };
+}
+
+describe("computeInventoryState", () => {
+  it("returns ACTIVE for a fresh refrigerated pen", () => {
+    expect(computeInventoryState(makeItem(), NOW_MS)).toBe("ACTIVE");
+  });
+
+  it("returns IN_USE once firstUseAt is set and within the 30-day window", () => {
+    const item = makeItem({
+      firstUseAt: new Date(NOW_MS - 5 * MS_PER_DAY),
+    });
+    expect(computeInventoryState(item, NOW_MS)).toBe("IN_USE");
+  });
+
+  it("returns EXPIRED when the in-use clock blew past 30 days", () => {
+    const item = makeItem({
+      firstUseAt: new Date(NOW_MS - 31 * MS_PER_DAY),
+    });
+    expect(computeInventoryState(item, NOW_MS)).toBe("EXPIRED");
+  });
+
+  it("returns EXPIRED when printed expiry has lapsed (unopened pen)", () => {
+    const item = makeItem({
+      printedExpiry: new Date(NOW_MS - 1 * MS_PER_DAY),
+    });
+    expect(computeInventoryState(item, NOW_MS)).toBe("EXPIRED");
+  });
+
+  it("returns EXPIRED when printed expiry has lapsed even if in-use clock still valid", () => {
+    const item = makeItem({
+      firstUseAt: new Date(NOW_MS - 5 * MS_PER_DAY),
+      printedExpiry: new Date(NOW_MS - 1 * MS_PER_DAY),
+    });
+    expect(computeInventoryState(item, NOW_MS)).toBe("EXPIRED");
+  });
+
+  it("returns USED_UP when dosesRemaining is zero (terminal — outranks EXPIRED)", () => {
+    const item = makeItem({
+      dosesRemaining: 0,
+      firstUseAt: new Date(NOW_MS - 31 * MS_PER_DAY),
+    });
+    expect(computeInventoryState(item, NOW_MS)).toBe("USED_UP");
+  });
+
+  it("returns USED_UP even when printed expiry has lapsed (dosesRemaining wins)", () => {
+    const item = makeItem({
+      dosesRemaining: 0,
+      printedExpiry: new Date(NOW_MS - 5 * MS_PER_DAY),
+    });
+    expect(computeInventoryState(item, NOW_MS)).toBe("USED_UP");
+  });
+
+  it("respects a custom in-use window override", () => {
+    // Ozempic — 56 days per its EPAR.
+    const item = makeItem({
+      firstUseAt: new Date(NOW_MS - 45 * MS_PER_DAY),
+    });
+    expect(computeInventoryState(item, NOW_MS, 56)).toBe("IN_USE");
+    expect(computeInventoryState(item, NOW_MS, 30)).toBe("EXPIRED");
+  });
+
+  it("treats the exact 30-day boundary as still IN_USE", () => {
+    const item = makeItem({
+      firstUseAt: new Date(NOW_MS - 30 * MS_PER_DAY),
+    });
+    expect(computeInventoryState(item, NOW_MS)).toBe("IN_USE");
+  });
+});
+
+describe("computeExpiresAt", () => {
+  it("returns null when neither firstUseAt nor printedExpiry is set", () => {
+    expect(computeExpiresAt(null, null)).toBeNull();
+  });
+
+  it("returns the printed expiry when only printedExpiry is set", () => {
+    const printed = new Date("2027-01-15T00:00:00Z");
+    expect(computeExpiresAt(null, printed)).toEqual(printed);
+  });
+
+  it("returns firstUseAt + 30 days when only firstUseAt is set", () => {
+    const firstUse = new Date("2026-05-01T00:00:00Z");
+    const result = computeExpiresAt(firstUse, null);
+    expect(result?.getTime()).toBe(firstUse.getTime() + 30 * MS_PER_DAY);
+  });
+
+  it("returns the in-use deadline when it lands before printed expiry", () => {
+    const firstUse = new Date("2026-05-01T00:00:00Z");
+    const printed = new Date("2027-01-15T00:00:00Z");
+    const result = computeExpiresAt(firstUse, printed);
+    expect(result?.getTime()).toBe(firstUse.getTime() + 30 * MS_PER_DAY);
+  });
+
+  it("returns the printed expiry when it lands before the in-use deadline", () => {
+    const firstUse = new Date("2026-05-01T00:00:00Z");
+    const printed = new Date("2026-05-10T00:00:00Z");
+    const result = computeExpiresAt(firstUse, printed);
+    expect(result).toEqual(printed);
+  });
+});
+
+describe("decrementDose", () => {
+  it("first_use — flips ACTIVE → IN_USE and sets firstUseAt", () => {
+    const item = makeItem();
+    const result = decrementDose(item, NOW);
+
+    expect(result.change).toBe("first_use");
+    expect(result.item.state).toBe("IN_USE");
+    expect(result.item.firstUseAt).toEqual(NOW);
+    expect(result.item.dosesRemaining).toBe(3);
+    expect(result.item.expiresAt?.getTime()).toBe(
+      NOW_MS + DEFAULT_IN_USE_WINDOW_DAYS * MS_PER_DAY,
+    );
+  });
+
+  it("consumed — IN_USE pen with doses left stays IN_USE", () => {
+    const firstUse = new Date(NOW_MS - 3 * MS_PER_DAY);
+    const item = makeItem({
+      state: "IN_USE",
+      dosesRemaining: 3,
+      firstUseAt: firstUse,
+    });
+    const result = decrementDose(item, NOW);
+
+    expect(result.change).toBe("consumed");
+    expect(result.item.state).toBe("IN_USE");
+    expect(result.item.firstUseAt).toEqual(firstUse);
+    expect(result.item.dosesRemaining).toBe(2);
+  });
+
+  it("depleted — final dose flips state to USED_UP", () => {
+    const firstUse = new Date(NOW_MS - 3 * MS_PER_DAY);
+    const item = makeItem({
+      state: "IN_USE",
+      dosesRemaining: 1,
+      firstUseAt: firstUse,
+    });
+    const result = decrementDose(item, NOW);
+
+    expect(result.change).toBe("depleted");
+    expect(result.item.state).toBe("USED_UP");
+    expect(result.item.dosesRemaining).toBe(0);
+  });
+
+  it("does not mutate its input", () => {
+    const firstUse = new Date(NOW_MS - 3 * MS_PER_DAY);
+    const item = makeItem({
+      state: "IN_USE",
+      dosesRemaining: 3,
+      firstUseAt: firstUse,
+    });
+    const snapshot = JSON.stringify(item);
+    decrementDose(item, NOW);
+    expect(JSON.stringify(item)).toBe(snapshot);
+  });
+
+  it("clamps dosesRemaining at zero even if called on an already-empty pen", () => {
+    const item = makeItem({
+      state: "USED_UP",
+      dosesRemaining: 0,
+      firstUseAt: new Date(NOW_MS - 5 * MS_PER_DAY),
+    });
+    const result = decrementDose(item, NOW);
+
+    expect(result.item.dosesRemaining).toBe(0);
+    expect(result.item.state).toBe("USED_UP");
+  });
+});
+
+describe("daysRemainingInUse", () => {
+  it("returns null for an ACTIVE (unopened) pen", () => {
+    expect(daysRemainingInUse(makeItem(), NOW_MS)).toBeNull();
+  });
+
+  it("returns null for an EXPIRED pen (clock blew)", () => {
+    const item = makeItem({
+      firstUseAt: new Date(NOW_MS - 31 * MS_PER_DAY),
+    });
+    expect(daysRemainingInUse(item, NOW_MS)).toBeNull();
+  });
+
+  it("returns null for a USED_UP pen", () => {
+    const item = makeItem({
+      dosesRemaining: 0,
+      firstUseAt: new Date(NOW_MS - 5 * MS_PER_DAY),
+    });
+    expect(daysRemainingInUse(item, NOW_MS)).toBeNull();
+  });
+
+  it("returns 25 days when the pen was opened 5 days ago", () => {
+    const item = makeItem({
+      firstUseAt: new Date(NOW_MS - 5 * MS_PER_DAY),
+    });
+    expect(daysRemainingInUse(item, NOW_MS)).toBe(25);
+  });
+
+  it("returns 0 on the last day of the window (still IN_USE)", () => {
+    // 29 days + 23h59m elapsed — under one day left, floor → 0.
+    const item = makeItem({
+      firstUseAt: new Date(NOW_MS - 30 * MS_PER_DAY + 60_000),
+    });
+    expect(daysRemainingInUse(item, NOW_MS)).toBe(0);
+  });
+});
diff --git a/src/lib/medications/inventory/state-machine.ts b/src/lib/medications/inventory/state-machine.ts
new file mode 100644
index 000000000..d49a4ca5c
--- /dev/null
+++ b/src/lib/medications/inventory/state-machine.ts
@@ -0,0 +1,224 @@
+/**
+ * v1.4.25 W19b — pen / vial inventory state machine.
+ *
+ * Pure functions over a minimal `InventoryItemView` shape — no DB
+ * access, no Prisma imports. The route handlers and the daily expire
+ * cron both compose this module with their own persistence layer.
+ *
+ * Decision tree (in evaluation order):
+ *   1. dosesRemaining === 0           → USED_UP   (terminal)
+ *   2. printedExpiry  && now > expiry → EXPIRED   (printed label trumps clock)
+ *   3. firstUseAt && now > firstUseAt + IN_USE_WINDOW_MS → EXPIRED
+ *   4. firstUseAt is set              → IN_USE
+ *   5. otherwise                      → ACTIVE
+ *
+ * The window length is fixed at 30 days per EMA EPAR §6.3 for
+ * Mounjaro KwikPen / Saxenda / Trulicity. Ozempic ships with a 56-day
+ * window per its own EPAR — that drug-specific override is *not*
+ * encoded here yet. The W19b scope deliberately uses the
+ * conservative 30-day default; a follow-up can read the
+ * drug-knowledge layer (`glp1-knowledge.ts`) and parametrise the
+ * window per Glp1DrugId. The state machine itself remains
+ * window-agnostic — it accepts the window length as input.
+ */
+
+import type { MedicationInventoryState } from "@/generated/prisma/client";
+
+/** Default in-use window per EMA EPAR §6.3 for the strictest GLP-1
+ *  agonists (Mounjaro, Saxenda, Trulicity). 30 days. */
+export const DEFAULT_IN_USE_WINDOW_DAYS = 30;
+
+const MS_PER_DAY = 24 * 60 * 60 * 1000;
+
+/**
+ * The subset of `MedicationInventoryItem` the state machine needs.
+ * Decoupling here means tests don't have to construct a full Prisma
+ * row, and the helper composes cleanly with the route layer's
+ * partial-update shapes.
+ */
+export interface InventoryItemView {
+  state: MedicationInventoryState;
+  dosesTotal: number;
+  dosesRemaining: number;
+  firstUseAt: Date | null;
+  printedExpiry: Date | null;
+}
+
+/**
+ * Resolve the canonical state for a pen / vial given the wall clock.
+ *
+ * The function is pure — given the same input it always returns the
+ * same state — so the caller can re-evaluate as often as it wants
+ * without side-effects. The expire-cron and the intake-hook both use
+ * this same evaluator; the only difference is what they do with the
+ * result.
+ */
+export function computeInventoryState(
+  item: InventoryItemView,
+  nowMs: number,
+  inUseWindowDays: number = DEFAULT_IN_USE_WINDOW_DAYS,
+): MedicationInventoryState {
+  // (1) Terminal — once a pen is used up, it stays used up. The
+  // printed expiry on an empty pen is irrelevant; the EXPIRED state
+  // exists to warn the user not to inject from a pen that's still
+  // got doses but has gone stale, and an empty pen has neither
+  // problem.
+  if (item.dosesRemaining <= 0) {
+    return "USED_UP";
+  }
+
+  // (2) Printed expiry trumps the in-use clock. A pen whose carton
+  // expiry has lapsed is EXPIRED even if it was never opened.
+  if (item.printedExpiry && nowMs > item.printedExpiry.getTime()) {
+    return "EXPIRED";
+  }
+
+  // (3) In-use clock blew. firstUseAt + window < now ⇒ EXPIRED.
+  if (item.firstUseAt) {
+    const inUseDeadlineMs =
+      item.firstUseAt.getTime() + inUseWindowDays * MS_PER_DAY;
+    if (nowMs > inUseDeadlineMs) {
+      return "EXPIRED";
+    }
+    // (4) Has been opened but still inside the window.
+    return "IN_USE";
+  }
+
+  // (5) Refrigerated, unopened.
+  return "ACTIVE";
+}
+
+/**
+ * Compute the `expiresAt` column. It's the min of the in-use
+ * deadline (firstUseAt + window) and the printed expiry — whichever
+ * lands first. If the pen has never been opened we fall back to the
+ * printed expiry alone (in-use clock hasn't started). If neither is
+ * set we return null so the daily expire-cron has nothing to scan.
+ */
+export function computeExpiresAt(
+  firstUseAt: Date | null,
+  printedExpiry: Date | null,
+  inUseWindowDays: number = DEFAULT_IN_USE_WINDOW_DAYS,
+): Date | null {
+  const inUseDeadline = firstUseAt
+    ? new Date(firstUseAt.getTime() + inUseWindowDays * MS_PER_DAY)
+    : null;
+
+  if (inUseDeadline && printedExpiry) {
+    return inUseDeadline.getTime() < printedExpiry.getTime()
+      ? inUseDeadline
+      : printedExpiry;
+  }
+  return inUseDeadline ?? printedExpiry ?? null;
+}
+
+/**
+ * Outcome of decrementing a dose. `first_use` fires when a pen
+ * transitions from ACTIVE → IN_USE (this intake event was its first
+ * use). `consumed` is the routine case. `depleted` fires on the
+ * intake that brings dosesRemaining to zero.
+ */
+export type DecrementOutcome = "first_use" | "consumed" | "depleted";
+
+/**
+ * Result of `decrementDose`. The returned `item` is a fresh object —
+ * the function does not mutate its input. The caller persists the
+ * returned shape (typically via a `prisma.medicationInventoryItem.update`).
+ */
+export interface DecrementResult {
+  item: {
+    state: MedicationInventoryState;
+    dosesRemaining: number;
+    firstUseAt: Date | null;
+    expiresAt: Date | null;
+  };
+  change: DecrementOutcome;
+}
+
+/**
+ * Consume one dose from a pen.
+ *
+ * If the pen is ACTIVE (never opened), this intake event sets
+ * `firstUseAt = intakeAt` and starts the 30-day clock. If the pen
+ * was already IN_USE, only `dosesRemaining` decrements. When the
+ * last dose is taken, the state moves to USED_UP and `expiresAt`
+ * is left untouched (the in-use window is no longer meaningful for
+ * an empty pen, but persisting the deadline preserves the audit
+ * trail).
+ *
+ * The function is defensive — if called on an EXPIRED or USED_UP
+ * pen it still decrements (caller's job to gate). The state
+ * recomputation respects whatever the final dose count + clock say.
+ */
+export function decrementDose(
+  item: InventoryItemView,
+  intakeAt: Date,
+  inUseWindowDays: number = DEFAULT_IN_USE_WINDOW_DAYS,
+): DecrementResult {
+  const wasFirstUse = item.firstUseAt === null;
+  const nextFirstUseAt = item.firstUseAt ?? intakeAt;
+  const nextDosesRemaining = Math.max(0, item.dosesRemaining - 1);
+  const nextExpiresAt = computeExpiresAt(
+    nextFirstUseAt,
+    item.printedExpiry,
+    inUseWindowDays,
+  );
+
+  const projected: InventoryItemView = {
+    state: item.state,
+    dosesTotal: item.dosesTotal,
+    dosesRemaining: nextDosesRemaining,
+    firstUseAt: nextFirstUseAt,
+    printedExpiry: item.printedExpiry,
+  };
+  const nextState = computeInventoryState(
+    projected,
+    intakeAt.getTime(),
+    inUseWindowDays,
+  );
+
+  const change: DecrementOutcome =
+    nextDosesRemaining === 0
+      ? "depleted"
+      : wasFirstUse
+        ? "first_use"
+        : "consumed";
+
+  return {
+    item: {
+      state: nextState,
+      dosesRemaining: nextDosesRemaining,
+      firstUseAt: nextFirstUseAt,
+      expiresAt: nextExpiresAt,
+    },
+    change,
+  };
+}
+
+/**
+ * Whole-days remaining in the 30-day in-use window, rounded toward
+ * zero. Returns null when the pen is not currently IN_USE (the UI
+ * surface for ACTIVE / EXPIRED / USED_UP shows a different label).
+ *
+ * The day count is computed against the in-use deadline only —
+ * the printed expiry is its own separate countdown surfaced as the
+ * "Carton expires {date}" label. This split avoids the UI lying to
+ * the user when the printed expiry is more imminent than the
+ * in-use deadline (the EXPIRED transition is still correct via
+ * `computeInventoryState`, but the day-count chip shows the
+ * 30-day-clock number).
+ */
+export function daysRemainingInUse(
+  item: InventoryItemView,
+  nowMs: number,
+  inUseWindowDays: number = DEFAULT_IN_USE_WINDOW_DAYS,
+): number | null {
+  if (!item.firstUseAt) return null;
+  const state = computeInventoryState(item, nowMs, inUseWindowDays);
+  if (state !== "IN_USE") return null;
+  const deadlineMs =
+    item.firstUseAt.getTime() + inUseWindowDays * MS_PER_DAY;
+  const remainingMs = deadlineMs - nowMs;
+  if (remainingMs <= 0) return 0;
+  return Math.floor(remainingMs / MS_PER_DAY);
+}