What is warfarin?
Warfarin is an anticoagulant (blood thinner) that helps prevent blood clots from forming. It’s prescribed for people at risk of stroke, particularly those with atrial fibrillation, or those who’ve had blood clots previously.
Warfarin is an oral medication typically taken once daily in the evening, and requires regular blood test monitoring to ensure it’s working effectively and safely.
Warfarin works by interfering with vitamin K, which your body uses to make clotting factors.
By reducing your blood’s ability to clot, warfarin significantly lowers your risk of dangerous clots that could cause strokes or other serious complications.
The effectiveness of warfarin is measured using the INR (International Normalised Ratio).
Most people taking warfarin aim for an INR between 2.0 and 3.0, meaning their blood takes 2-3 times longer to clot than normal.
Research shows that about 60% of people taking warfarin have atrial fibrillation, with an estimated 470,000+ patients receiving anticoagulation in the UK.
What is Mounjaro?
Mounjaro is a dual-action weight-loss injection that mimics two hormones in the body that help us feel fuller and lose weight.
The drug in Mounjaro is tirzepatide, which is a synthetic mimic of GLP-1 and GIP. GLP-1 and GIP are hormones released by the gut after we eat to help us feel full and manage our blood sugar levels.
Mounjaro is a long-acting medication injected once a week. It takes 4-5 weeks for the drug to reach a steady state in the body, where the levels in the blood remain consistent and the drug’s effects become more pronounced.
Research suggests Mounjaro supports weight loss of up to 26% after two years, which is around 10% more weight loss than another well-known weight-loss injection, Wegovy.
Why would you take Mounjaro alongside warfarin?
If you’re taking warfarin for heart conditions like atrial fibrillation and also have obesity, you may benefit from taking Mounjaro to support weight loss alongside your anticoagulation therapy.
Obesity can increase the risk of blood clots and cardiovascular problems, so losing weight while maintaining proper anticoagulation could provide dual benefits for your heart health.
The interaction occurs because Mounjaro slows stomach emptying, which can affect how quickly warfarin is absorbed into your bloodstream. This doesn’t make warfarin less effective, but it can temporarily change the timing of its effects.
Your local healthcare team will determine how frequently to check your INR based on your response, but expect more frequent monitoring when starting Mounjaro or increasing doses.
Here’s a list of things to consider when taking warfarin with Mounjaro:
- You should consult your anticoagulation clinic and GP before starting Mounjaro
- Your INR will need to be monitored more frequently, especially when starting Mounjaro or increasing doses
- The interaction typically stabilises after several weeks on a steady Mounjaro dose
- Watch for signs of unusual bleeding (bruising, nosebleeds, blood in urine) and report them immediately
- Never adjust your warfarin dose yourself; always follow your anticoagulation clinic’s guidance
- Inform all healthcare providers that you take both medications
According to UK medical guidance from the Medway and Swale NHS Formulary, this combination requires ‘close monitoring’ rather than avoiding the combination entirely.
However, there is no extensive research specifically investigating the combination of Mounjaro with warfarin, and some individuals may experience more significant INR changes, requiring dose adjustments.
If you’re signing up for Second Nature’s Mounjaro programme, the clinical team will discuss and confirm this, but in general, if you’re taking warfarin, you will be able to take Mounjaro with enhanced monitoring from your local healthcare team.
If you do start taking warfarin with Mounjaro, we recommend letting your GP know.
Common questions answered
How often will I need blood tests when taking both medications?
Your anticoagulation clinic will increase monitoring frequency when starting Mounjaro or adjusting doses.
The exact schedule depends on your individual response and clinic protocols.
What are the warning signs I should watch for?
Unusual bruising from minor bumps, persistent nosebleeds, blood in urine (pink or red colour), or dark tarry stools.
These suggest your INR might be too high and you need immediate medical attention. Severe headaches or abdominal pain also require urgent assessment.
Can this interaction be dangerous?
With proper monitoring, the interaction is manageable. The risk comes from ignoring monitoring requirements, not from the combination itself.
Case reports show problems typically occur when monitoring isn’t increased appropriately.
Will Mounjaro stop my warfarin from working?
No, it may temporarily alter the timing of absorption, but your healthcare team can adjust the doses to maintain the effectiveness of the drug. The anticoagulant effect continues; it’s just the timing that may change initially.
Should I separate the timing of taking these medications?
Current guidance doesn’t recommend this, as Mounjaro’s effects on stomach emptying last throughout the week, not just immediately after injection. Continue taking warfarin at your usual time.
How long does the interaction last?
The absorption effects are most noticeable when starting Mounjaro or increasing doses, and tend to diminish once you’re on a stable Mounjaro regimen, though timing varies between individuals.
What if I need surgery while taking both medications?
Standard pre-operative protocols apply. Both medications may need temporary adjustment before surgery, but this is routine for anticoagulation clinics. Always inform your surgical team about both medications.
Are there patients who shouldn’t take this combination?
Those with poorly controlled INR levels, frequent bleeding episodes, or significant malnutrition may be at higher risk. Your healthcare team will assess your individual situation.
Related medication interactions
If you’re taking other medications alongside warfarin and considering Mounjaro, you might also be interested in:
- Managing multiple heart medications with weight loss injections
- Blood pressure medication interactions with GLP-1 treatments
- Coordinating diabetes medications with Mounjaro therapy
Take home message
Taking warfarin doesn’t prevent you from benefiting from Mounjaro for weight loss.
While it may require more monitoring from your healthcare team, it’s generally considered safe to take Mounjaro with warfarin.
Most people taking warfarin will be eligible for Mounjaro, but if you do start taking Mounjaro then it’ll be very important to let your healthcare team know so they can monitor your INR more frequently and adjust monitoring schedules if needed.
Second Nature's Mounjaro programme
Second Nature provides Mounjaro as part of our Mounjaro weight-loss programme
Why choose Second Nature over other medication providers, assuming you’re eligible?
Because peace of mind matters.
We’ve had the privilege of working with the NHS for over eight years, helping people across the UK take meaningful steps toward a healthier, happier life.
Our programmes are designed to meet people where they are, whether that means support with weight loss through compassionate one-to-one health coaching, or access to the latest weight-loss medications (like Mounjaro) delivered alongside expert care from a multidisciplinary team of doctors, psychologists, dietitians, and personal trainers.
At the heart of everything we do is a simple belief: real, lasting change comes from building better habits, not relying on quick fixes. We’re here to support that change every step of the way.
With over a decade of experience, thousands of lives changed, and a long-standing record of delivering programmes used by the NHS, we believe we're the UK’s most trusted weight-loss programme.
We hope to offer you something invaluable: peace of mind, and the support you need to take that first step.
