The FDA-listed label for Mounjaro, the tirzepatide product, carried a boxed warning about thyroid C-cell tumors effective 27 August 2026. The warning states that tirzepatide caused dose-dependent and treatment-duration-dependent thyroid C-cell tumors in both male and female rats at clinically relevant exposures.

Background

Tirzepatide is identified in the supplied record as the active drug associated with Mounjaro. The label warning concerns thyroid C-cell tumors observed in rats. The supplied data do not provide additional information about tirzepatide’s mechanism, approved indications, dosing, contraindications, or the prevalence of the conditions it may treat.

Label change

The event was classified as an FDA label revision. The Mounjaro label included a boxed warning headed “Risk of Thyroid C-Cell Tumors.” The warning reported findings in both male and female rats and described the tumors as dependent on dose and treatment duration.

The label also stated that the rat findings occurred at clinically relevant exposures. It further stated that whether Mounjaro causes thyroid C-cell tumors in humans is unknown. The supplied record did not provide human tumor data, a quantified clinical risk, or additional conclusions about the warning.

What this means

The revised label formally places the thyroid C-cell tumor risk information in a boxed warning for Mounjaro. The available record supports a distinction between the animal findings described in the warning and the uncertainty about whether the same effect occurs in humans.

The data do not specify the number of tumors in the rat studies, the doses used, the exposure measurements, the duration of treatment, or any human safety estimate. They also do not state whether the revision changed Mounjaro’s indication, dosing instructions, or other prescribing requirements.

Source

Source: FDA, DailyMed drug-label document for Mounjaro, identified by set ID d2d7da5d-ad07-4228-955f-cf7e355c8cc0 and effective 27 August 2026. The document is available through the authoritative DailyMed host at dailymed.nlm.nih.gov.