Mounjaro still carries a boxed warning on thyroid C-cell tumors and an MTC contraindication

Mounjaro (tirzepatide) entered the U.S. market in 2022 with a boxed warning about thyroid C-cell tumors and a contraindication covering people with a personal or family history of medullary thyroid carcinoma (MTC) or Multiple Endocrine Neoplasia syndrome type 2. Both items are still present in the prescribing information dated August 27, 2026 ^[2].

The boxed warning states that tirzepatide causes thyroid C-cell tumors in rats, and that whether it causes such tumors, including MTC, in humans is unknown, since the relevance of the rodent findings to people has not been determined. It directs that patients be counseled about the possible MTC risk and about symptoms of thyroid tumors ^[2]. In the contraindications section, a personal or family history of MTC or MEN 2 appears alongside known serious hypersensitivity to tirzepatide or to the product's other ingredients ^[2]. Because family history is included, that restriction can apply to someone who has never had thyroid disease.

The label has been revised repeatedly in the years since. A pediatric indication, for patients 10 years and older with type 2 diabetes, was added in December 2025, and in August 2026 the drug gained an indication to reduce the risk of major adverse cardiovascular events in adults with type 2 diabetes who are at high risk for them ^[2]. The pediatric application, NDA 215866/S-039, was submitted June 19, 2025, taken up on a priority basis and completed November 26, 2025, ahead of a December 19, 2025 goal date, with a recommendation of approval ^[1]. The recent-major-changes table in the current label lists severe gastrointestinal adverse reactions (December 2025), diabetic retinopathy complications in patients with a history of the condition (August 2026), and a warning against sharing a Mounjaro KwikPen between patients (January 2026). It does not list the boxed warning or the MTC/MEN 2 contraindication ^[2].

Dosing starts at 2.5 mg injected under the skin once weekly, rising by 2.5 mg after at least four weeks at the current dose if more blood sugar control is needed. The maximum is 15 mg weekly for adults and 10 mg weekly for patients 10 and older ^[2]. Adverse reactions reported in at least 5% of patients are nausea, diarrhea, decreased appetite, vomiting, constipation, dyspepsia and abdominal pain ^[2]. Other warnings and precautions cover pancreatitis, low blood sugar when the drug is used with insulin or an insulin secretagogue, serious allergic reactions, kidney injury from volume depletion, gallbladder disease, and pulmonary aspiration during general anesthesia or deep sedation ^[2].

Why it matters for patients

The MTC/MEN 2 entry is a directive rather than a caution to weigh against benefits: the label lists it among the conditions in which Mounjaro is not to be used ^[2]. Since family history counts, people with no thyroid problems of their own can fall under it ^[2].

The question behind the boxed warning — whether tirzepatide can produce these tumors in people — is still open in the label, which says human relevance has not been established ^[2]. Patients are to be told about the potential MTC risk and about thyroid tumor symptoms ^[2].

For anyone comparing tirzepatide with other treatments, these two items have carried through every revision shown in the current prescribing information, including the August 2026 update, rather than being softened or dropped ^[2].

What happens next

No timeline has been disclosed for reviewing, modifying or removing the boxed warning or the MTC/MEN 2 contraindication ^[2]. The label is revised as needed, and the current version is dated August 27, 2026 ^[2]. Whether the rat tumor finding has a counterpart in humans remains unresolved in the label itself ^[2].