Mounjaro and warfarin: interactions and safety advice

If you're thinking about losing weight with Mounjaro injections, it's important to let your healthcare team know about any medications you're currently taking.

That's because medications can interfere with each other. One medication can affect how the other is absorbed into the body, change how it works or lead to unpleasant (or even dangerous) side effects. When this happens, we call it a 'drug interaction'.

In this guide, we look at potential interactions between Mounjaro and the blood-thinning medication warfarin.

What is warfarin?

Warfarin belongs to a class of medications called 'coumarins'. Coumarins are anticoagulants – commonly known as 'blood thinners'. Medications like these help to prevent blood clots in the body.

Your doctor might recommend that you take warfarin if you've had a blood clot or are at risk of developing them in future. That includes people who have:¹

• Had a blood clot in the leg (deep vein thrombosis)
• Had a blood clot in the lungs (pulmonary embolism)
• Had an operation to replace a heart valve
• Thrombophilia or another blood-clotting disorder
• An irregular heartbeat (atrial fibrillation)
  
  

You take warfarin as a tablet. Most people take it once daily, in the morning.

In the UK, doctors sometimes prescribe warfarin under the brand name 'Marevan'. If you're in the US, you might know it as 'Coumadin' or 'Jantoven'.

Do Mounjaro and warfarin interact?

The UK's National Institute for Health and Care Excellence (NICE) recommends that doctors closely monitor patients who take Mounjaro and warfarin together.²

That's based on advice from Mounjaro's manufacturer, Eli Lilly. In its information for doctors, the manufacturer states:

"Mounjaro delays gastric emptying and thereby has the potential to impact the absorption of concomitantly administered oral medications."³

In plain language, that means Mounjaro makes the stomach take longer to process food and tablets. Because you take warfarin as a tablet, Mounjaro might affect how it's absorbed and how well it works.

This is of particular concern for warfarin because it's a drug with a narrow therapeutic index. In other words, small changes in how the body absorbs the medication can significantly impact its safety and effectiveness.

Can I take Mounjaro and warfarin together?

While there's a risk of interaction between Mounjaro and warfarin, you still might be able to take them at the same time. It all depends on what your healthcare team thinks is safe and appropriate for you.

However, it's especially important that you let them know that you're taking warfarin before you start treatment with Mounjaro. This way, they can keep a close eye on potential interactions and make sure you stay safe throughout your treatment.

The same goes if it's the other way around – if you're already taking Mounjaro and your doctor recommends warfarin. For your safety and well-being, you should tell your doctor about your Mounjaro injections before they prescribe warfarin.

Semapen is a UK weight loss clinic founded by Professor David Kerrigan, former president of the British Obesity & Metabolic Specialist Society. For more tips, read our GLP-1 guides or follow our blog. 

Sources

1. NHS (2022) About warfarin. Retrieved from https://www.nhs.uk/medicines/warfarin/about-warfarin/

2. National Institute for Health and Care Excellence (n.d.) Tirzepatide: Interactions. Retrieved from https://bnf.nice.org.uk/interactions/tirzepatide/

3. Mounjaro (2026) [package insert]. Indianapolis, IN: Eli Lilly and Company. Retrieved from: https://uspl.lilly.com/mounjaro/mounjaro.html

This article was reviewed and approved by Lujain Alhassan, Bariatric Nutritionist, on 28 February 2026.