diff --git a/src/app/api/medications/[id]/route.ts b/src/app/api/medications/[id]/route.ts
index de6424398..fd576ea10 100644
--- a/src/app/api/medications/[id]/route.ts
+++ b/src/app/api/medications/[id]/route.ts
@@ -285,7 +285,10 @@ export const PUT = apiHandler(
     // below excludes every actioned row — and the previous cadence is the one
     // genuinely unrecoverable part, which is what C7 puts in the audit row.
     // The DELETE beside it stays refused.
-    const { user } = await requireRecordAuth("manage", "medications");
+    const { user, authMethod } = await requireRecordAuth(
+      "manage",
+      "medications",
+    );
 
     const { id } = await params;
     // v1.5.5 C-E3-3 — route ownership check through the shared helper
@@ -1061,7 +1064,24 @@ export const PUT = apiHandler(
       if (projected) medication = { ...medication, ...projected };
     }
 
-    if (category !== undefined) await setMedicationCategory(id, category);
+    // v1.40 (#1041) — an iPhone app build that predates custom categories
+    // decodes an unknown category as OTHER and sends it back on every edit,
+    // which would overwrite the person's own category with OTHER. A native
+    // client (Bearer) sending OTHER over a custom category keeps the custom
+    // one; moving a custom category back to OTHER stays possible on the web.
+    let categoryToWrite = category;
+    if (category === "OTHER" && authMethod === "bearer") {
+      const current = (await resolveMedicationCategories([id]))[id];
+      if (current?.category.startsWith("custom:")) {
+        categoryToWrite = undefined;
+        annotate({
+          action: { name: "medication.update.custom_category_kept" },
+          meta: { entity_id: id },
+        });
+      }
+    }
+    if (categoryToWrite !== undefined)
+      await setMedicationCategory(id, categoryToWrite);
     const resolvedCategory = (await resolveMedicationCategories([id]))[id];
     const normalizedCategory = resolvedCategory?.category ?? "OTHER";
 
diff --git a/tests/integration/medication-custom-categories.test.ts b/tests/integration/medication-custom-categories.test.ts
index 694359423..262ea7e75 100644
--- a/tests/integration/medication-custom-categories.test.ts
+++ b/tests/integration/medication-custom-categories.test.ts
@@ -192,6 +192,49 @@ describe("custom medication categories", () => {
     expect(body.data).toMatchObject({ category: key, categoryLabel: "Heart" });
   });
 
+  it("keeps a custom category when an older iPhone build sends OTHER back", async () => {
+    const key = await createCategory("Heart");
+    const created = await (await createMedication(key)).json();
+    const { hashToken } = await import("@/lib/auth/hmac");
+    const raw = "hlk_custom-category-native-0000";
+    await getPrismaClient().apiToken.create({
+      data: {
+        userId: OWNER,
+        name: "native",
+        tokenHash: hashToken(raw),
+        permissions: ["*"],
+      },
+    });
+    cookieJar.clear();
+    headerJar.set("authorization", `Bearer ${raw}`);
+    const { PUT } = await import("@/app/api/medications/[id]/route");
+    const res = await PUT(
+      req("PUT", `/api/medications/${created.data.id}`, {
+        name: "Renamed",
+        category: "OTHER",
+      }),
+      { params: Promise.resolve({ id: created.data.id }) },
+    );
+    const body = await res.json();
+    expect(res.status, JSON.stringify(body)).toBe(200);
+    expect(body.data).toMatchObject({ category: key, categoryLabel: "Heart" });
+    headerJar.clear();
+    await signIn(OWNER);
+  });
+
+  it("moves a custom category back to OTHER when the web asks", async () => {
+    const key = await createCategory("Heart");
+    const created = await (await createMedication(key)).json();
+    const { PUT } = await import("@/app/api/medications/[id]/route");
+    const res = await PUT(
+      req("PUT", `/api/medications/${created.data.id}`, { category: "OTHER" }),
+      { params: Promise.resolve({ id: created.data.id }) },
+    );
+    const body = await res.json();
+    expect(res.status, JSON.stringify(body)).toBe(200);
+    expect(body.data.category).toBe("OTHER");
+  });
+
   it("renames, hides and caps", async () => {
     const key = await createCategory("Heart");
     const { PATCH } =