Mounjaro approved with a thyroid C-cell boxed warning

When the FDA cleared tirzepatide as Mounjaro for type 2 diabetes, the drug entered the US market with a boxed warning about thyroid C-cell tumors. The label lists an initial US approval year of 2022, and the warning — along with an absolute contraindication for certain patients — remains in place in the version revised in August 2026 ^[2].

What the warning says

The boxed warning is titled "WARNING: RISK OF THYROID C-CELL TUMORS." It states that "Tirzepatide causes thyroid C-cell tumors in rats," and that "It is unknown whether MOUNJARO causes thyroid C-cell tumors, including medullary thyroid carcinoma (MTC), in humans as the human relevance of tirzepatide-induced rodent thyroid C-cell tumors has not been determined" ^[2].

The second half of the warning is the part that changes who can be prescribed the drug at all: Mounjaro "is contraindicated in patients with a personal or family history of MTC or in patients with Multiple Endocrine Neoplasia syndrome type 2 (MEN 2)," and prescribers are told to counsel patients about the potential risk of MTC and the symptoms of thyroid tumors ^[2]. The contraindications section repeats that restriction, and adds known serious hypersensitivity to tirzepatide or any of the label's excipients ^[2].

Tirzepatide is described on the label as a glucose-dependent insulinotropic polypeptide (GIP) receptor and glucagon-like peptide-1 (GLP-1) receptor agonist — a dual-receptor drug rather than a GLP-1 alone ^[2][1]. The rat findings, not human cases, are what the boxed warning rests on, and the label is explicit that the human relevance has not been determined ^[2]. The specific rodent dose levels and tumor rates are not included in the source documents reviewed here.

A label that has grown around a warning that hasn't changed

Since approval, the Mounjaro label has picked up new indications and new safety language, but the boxed warning and the MTC/MEN 2 contraindication have stayed. Recent major changes listed on the label include a pediatric indication for patients 10 years and older dated 12/2025, a major adverse cardiovascular events indication dated 08/2026, new language on severe gastrointestinal adverse reactions dated 12/2025, diabetic retinopathy complications dated 08/2026, and a warning never to share a KwikPen between patients dated 01/2026 ^[2].

The pediatric expansion came through a priority-review supplemental application, NDA 215866/S-039, submitted by Eli Lilly on June 19, 2025, with a PDUFA goal date of December 19, 2025; FDA's review was completed November 26, 2025 and recommended approval for patients 10 and older with type 2 diabetes ^[1]. The maximum dosage for those pediatric patients is 10 mg once weekly, versus 15 mg once weekly for adults; both start at 2.5 mg once weekly with increases of 2.5 mg after at least 4 weeks on the current dose ^[2][1].

Other warnings on the current label include acute pancreatitis, hypoglycemia when used with insulin or an insulin secretagogue, serious hypersensitivity reactions such as anaphylaxis and angioedema, acute kidney injury from volume depletion, severe gastrointestinal reactions (not recommended in severe gastroparesis), acute gallbladder disease, and pulmonary aspiration during general anesthesia or deep sedation ^[2]. The most common adverse reactions reported in at least 5% of patients are nausea, diarrhea, decreased appetite, vomiting, constipation, dyspepsia and abdominal pain ^[2].

Why it matters for patients

A contraindication is stronger than a warning. Under the label, a personal or family history of medullary thyroid carcinoma, or a diagnosis of MEN 2, rules out tirzepatide entirely — not "use with caution" ^[2]. That is why intake forms and telehealth questionnaires ask about thyroid cancer in relatives, and why an incomplete family history can matter.

It also matters that the warning is built on animal data. The label itself says the human relevance is undetermined ^[2]. That means patients are weighing a risk that has not been quantified in people, alongside the drug's approved benefits in type 2 diabetes and, more recently, cardiovascular risk reduction in adults with type 2 diabetes at high risk ^[2].

Finally, the boxed warning has traveled with tirzepatide through every labeling update on the Mounjaro prescribing information seen here, including the pediatric and cardiovascular expansions ^[2]. The Zepbound label is not included in these sources, so its current text is not documented here.

What happens next

No change to the boxed warning appears in the current label, which was revised 8/2026 ^[2]. Key dated milestones on record: pediatric sNDA submitted June 19, 2025; FDA review completed November 26, 2025; PDUFA goal date December 19, 2025; pediatric indication reflected on the label 12/2025; cardiovascular indication 08/2026 ^[1][2].