European regulator opens review of possible tirzepatide-warfarin interaction
European regulators are reviewing four reports that tirzepatide, the drug in Mounjaro and Zepbound, may lower INR in people taking warfarin; no label change has been decided [1].
The European Medicines Agency's Pharmacovigilance Risk Assessment Committee has taken up a new safety signal on a possible interaction between tirzepatide and warfarin, according to the minutes of its 29 September to 2 October 2025 meeting [1]. The minutes, published on 14 November 2025, describe the signal as an interaction between tirzepatide and warfarin and other coumarin derivatives leading to a decreased international normalized ratio, or INR [1].
Tirzepatide is the molecule sold as Mounjaro and Zepbound. The signal was raised by Spain and is based on four cases [1]. The minutes do not describe those cases: they give no ages, doses, or how long the patients had taken either medicine, and they do not say whether any of the four had bleeding or clotting problems [1].
The committee treated the reports as a signal, meaning they were flagged for review. The minutes do not state that a cause-and-effect link between tirzepatide and the INR change has been established [1].
The committee told the marketing authorisation holder, Eli Lilly, to submit a cumulative review of the issue within 90 days and to discuss whether the product information for tirzepatide medicines should be updated [1]. No decision on labelling has been made. The minutes note that they are a working document and that some information is withheld as commercially confidential or because procedures are still open [1]. The minutes also do not mention any action by the US Food and Drug Administration on this question, and no such action has been reported.
Why it matters for patients
Warfarin is a blood thinner, and INR is the blood test used to monitor how strongly it is working. The signal concerns a drop in that test result in people taking both medicines [1]. A lower INR can indicate that warfarin is having less of its expected effect, but the minutes do not say how far INR fell in the four cases, how often this happened, or whether it caused any harm [1].
Tirzepatide is used for type 2 diabetes and for weight loss in the United States, and some people who take it also take warfarin for conditions such as atrial fibrillation or a prior blood clot. The PRAC document contains no recommendation for patients, no instruction to stop or change either medicine, and no product recall [1]. It also does not say whether US prescribing information for tirzepatide already addresses warfarin.
The review is at an early stage. A signal is a report that regulators decide to examine, not a finding that a medicine caused a problem, and the outcome of this one has not been decided [1].
What happens next
The 90-day deadline for Lilly's cumulative review runs from the committee's 29 September to 2 October 2025 meeting, which points to a submission around late December 2025 or early January 2026; the minutes do not give an exact due date [1]. After that review, the committee is expected to consider whether changes to the product information are needed, but no timeline for that decision is stated [1].
The EMA says additional details on some of these procedures will appear in its PRAC meeting highlights once the procedures are finalised [1]. Until then, whether the interaction is confirmed, how it might be managed, and whether regulators outside Europe will act have not been reported.
Sources
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