EMA opens a tirzepatide warfarin interaction signal

Europe's drug safety committee has opened a new safety signal for tirzepatide, the drug sold as Mounjaro and Zepbound, over a possible interaction with warfarin and related coumarin blood thinners. The issue came up at the European Medicines Agency's Pharmacovigilance Risk Assessment Committee (PRAC) meeting held September 29 to October 2, 2025 ^[1].

Spain brought the signal to the committee after reviewing four cases in which patients taking tirzepatide alongside warfarin or another coumarin derivative saw their international normalised ratio (INR) drop ^[1]. INR is the standard blood test used to check how well warfarin is thinning a patient's blood. A falling INR in someone on warfarin can mean the drug is not working as well as it should, which raises the risk of unwanted blood clots ^[1].

The PRAC did not conclude that tirzepatide causes this problem. Instead, the committee required Eli Lilly, which makes tirzepatide, to submit a cumulative review of all available data within 90 days ^[1]. That review will need to weigh whether the four Spanish cases, together with any other reports Lilly has on file, add up to a real signal worth acting on. The committee also asked Lilly to address whether the product information for tirzepatide should be updated to reflect this possible interaction ^[1].

Why it matters for patients

Warfarin is a widely used blood thinner that requires regular INR monitoring to keep dosing in a safe range. If tirzepatide turns out to lower INR in some patients, that could mean warfarin becomes less effective while both drugs are taken together, which in theory could raise the risk of clotting problems such as stroke or deep vein thrombosis ^[1]. At this stage, the EMA has only opened a signal for review — it has not confirmed the interaction, changed any warnings, or advised prescribers to alter how the drugs are used together ^[1].

Patients in the United States taking Mounjaro or Zepbound who are also on warfarin are not affected by any label change right now, because no such change has happened yet. The source material describes an EU regulatory process, and it does not say whether the FDA is reviewing the same signal ^[1]. Anyone on both a coumarin-type blood thinner and tirzepatide who has questions about their own monitoring should raise them with the clinician managing their anticoagulation, since INR checks are already a routine part of warfarin care.

It is also not yet known, based on the available minutes, how strong the evidence is behind the four Spanish cases, whether other countries have reported similar cases, or what mechanism might explain a link between tirzepatide and reduced anticoagulant effect ^[1]. The PRAC minutes describe the administrative step of opening the signal and setting a deadline for Lilly's response, but they do not include the underlying case reports or a scientific explanation ^[1].

What happens next

Lilly is expected to submit its cumulative review of the data to the PRAC within 90 days of the September 29–October 2, 2025 meeting, putting a rough deadline near late December 2025 or early January 2026 ^[1]. That submission is meant to include a recommendation on whether tirzepatide's product information should be updated to mention a possible interaction with warfarin and other coumarin derivatives ^[1]. The PRAC will then need to review that submission and decide whether further regulatory action, such as a label update in the European Union, is warranted. The source minutes do not specify a date for that follow-up decision ^[1].