The U.S. Food and Drug Administration-listed Zepbound label for tirzepatide carried a boxed warning effective August 28, 2026. The warning states that tirzepatide caused thyroid C-cell tumors in rats, with risk related to dose and treatment duration at clinically relevant exposures.
The label also states that it is unknown whether Zepbound causes thyroid C-cell tumors in humans. The supplied record identifies this as an FDA label revision and does not provide additional clinical data about human thyroid tumor risk.
Background
Tirzepatide is listed in the supplied record under the brand names Zepbound and Mounjaro, as well as Zepbound KwikPen and Mounjaro KwikPen. The event concerns the Zepbound label. The available data do not specify tirzepatide’s drug class, mechanism, approved indications, dosing, or the prevalence of any related condition.
What changed
The key label information is a boxed warning headed “Risk of Thyroid C-Cell Tumors.” It reports that, in rats, tirzepatide caused thyroid C-cell tumors in a manner dependent on both dose and treatment duration. The warning describes these findings at clinically relevant exposures.
The label distinguishes the animal findings from the current state of knowledge in humans: whether Zepbound causes thyroid C-cell tumors, including in people, is unknown based on the supplied record.
What this means
The revision places the thyroid C-cell tumor concern prominently in the Zepbound prescribing information. For clinicians, researchers, patient advocates, and editors reviewing the change, the central documented fact is the presence of the boxed warning and its basis in rat findings.
The record does not include human incidence estimates, participant-level safety results, comparative analyses, p-values, confidence intervals, or evidence establishing whether Zepbound causes thyroid C-cell tumors in humans. It therefore supports describing the human risk as unknown, rather than assigning a numerical risk or drawing a conclusion about causation in people.
Source
Source: FDA, DailyMed label document for Zepbound, with the boxed warning effective August 28, 2026. The authoritative document is available through dailymed.nlm.nih.gov at dailymed.nlm.nih.gov/dailymed/lookup.cfm?setid=487cd7e7-434c-4925-99fa-aa80b1cc776b.