The UK's most complete GLP-1 guide
Every Question Answered. Every Side Effect Explained.
- How GLP-1 medications work
- Mounjaro vs Wegovy compared
- Injection instructions
- Side effects & solutions
- What to eat on treatment
- FAQs answered
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Mounjaro and Warfarin: What You Need to Know
If you take warfarin and are considering Mounjaro (tirzepatide) for weight management, it's important to understand how these medications interact. Mounjaro may affect how your body absorbs other medicines, including warfarin, which is a blood thinner used to prevent blood clots. This means your GP or prescriber needs to assess your individual situation carefully before starting treatment. This guide explains the interaction, compares Mounjaro with Wegovy, and helps you understand what your prescriber will consider when deciding if Mounjaro is suitable for you.
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Enter your height and weight below. Supports both imperial (stone & lbs) and metric (kg). Results include NHS category, healthy weight range, and ethnicity adjustments.
What Is BMI and Why Does It Matter?
Body Mass Index (BMI) is the most widely used screening tool for assessing whether your weight falls within a healthy range. Used by the NHS, GPs, and weight management services across the UK, BMI provides a quick snapshot of your weight relative to your height. The formula is simple: your weight in kilograms divided by your height in metres squared (kg/m²).
While BMI isn't a perfect measure — it doesn't account for muscle mass, bone density, or fat distribution — it remains the standard first step in identifying potential weight-related health risks including type 2 diabetes, cardiovascular disease, high blood pressure, and certain cancers.
The NHS also recommends adjusted thresholds for people of South Asian, Chinese, or Black African heritage — a BMI of 23 or above is classified as overweight, and 27.5 as obese. Our calculator includes this adjustment as a one-click toggle.
How Mounjaro May Affect Warfarin and Other Medicines
Mounjaro is a dual GIP and GLP-1 receptor agonist — a once-weekly weight loss injection licensed in the UK. One important property of this medication is that it slows gastric emptying, meaning food (and oral medicines) move more slowly through your stomach and digestive system. This can affect how quickly your body absorbs medications taken by mouth.
Warfarin is an oral anticoagulant that requires stable, consistent absorption for your INR (blood clotting measure) to remain in the safe therapeutic range. If Mounjaro delays warfarin absorption, your INR could become unpredictable, potentially increasing bleeding risk or clot risk depending on the change.
- Your prescriber will need to know you take warfarin before starting Mounjaro
- You may need more frequent INR monitoring if you start Mounjaro
- Your warfarin dose may need adjustment by your anticoagulation clinic
- Never stop warfarin or change Mounjaro without medical guidance
For this reason, your GP or anticoagulation clinic must be involved in any decision to start Mounjaro. This is not a contraindication, but it does require careful management and coordination between your weight management prescriber and your anticoagulation specialist.
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Three simple steps to understand your weight and take action.
Enter your height and weight in our free calculator. Get your BMI, NHS category, healthy weight range, and ethnicity-adjusted thresholds instantly.
If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
Use our progress tracker to log your weight weekly, visualise your journey with charts, and stay motivated — all stored privately in your browser.
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Mounjaro vs Wegovy: Comparison for Warfarin Users
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are GLP-1-based weight loss injections licensed in the UK, but they work slightly differently. Wegovy is a GLP-1 agonist only, whilst Mounjaro adds a dual GIP + GLP-1 mechanism. Both slow gastric emptying, so both may theoretically affect warfarin absorption similarly.
The choice between them for someone taking warfarin depends on:
- Your prescriber's experience with each medication in anticoagulated patients
- Clinical trial data — both have evidence of effectiveness, though results vary individually
- Your INR stability history and anticoagulation clinic's confidence in monitoring
- Any other health conditions you have
Neither is inherently "safer" with warfarin — what matters is close coordination between your weight management and anticoagulation teams, plus regular INR monitoring. Use our free BMI calculator to check your current weight status, and discuss all options with your prescriber.
- Dual GIP/GLP-1 receptor agonist
- Up to 22.5% body weight loss (SURMOUNT-1)
- Weekly injection, dose escalation over 20+ weeks
- Available via private prescription in the UK
- Doses: 2.5mg → 5mg → 7.5mg → 10mg → 12.5mg → 15mg
- GLP-1 receptor agonist
- Up to 15–17% body weight loss (STEP trials)
- Weekly injection, 5-step dose escalation
- NHS & private prescription available
- Doses: 0.25mg → 0.5mg → 1mg → 1.7mg → 2.4mg
Mounjaro vs Wegovy: Head-to-Head
A side-by-side comparison of the two most popular GLP-1 weight loss medications available in the UK.
| Feature | Mounjaro | Wegovy |
|---|---|---|
| Drug class | Dual GIP/GLP-1 agonist | GLP-1 agonist |
| Active ingredient | Tirzepatide | Semaglutide |
| Avg. weight loss | 15–22.5% | 12–17% |
| Key trial | SURMOUNT-1 (2022) | STEP 1 (2021) |
| Max dose | 15 mg weekly | 2.4 mg weekly |
| Escalation period | 20–32 weeks | 16–20 weeks |
| Administration | Weekly injection (pen) | Weekly injection (pen) |
| NHS availability | Private prescription only | NHS & private |
| Approx. private cost | £150–£250/month | £150–£300/month |
| Common side effects | Nausea, diarrhoea, constipation | Nausea, vomiting, diarrhoea |
Data sourced from SURMOUNT-1 and STEP 1 clinical trials. Costs are approximate and vary by provider. Always consult a prescriber.
Am I Eligible for Weight Loss Medication?
In the UK, eligibility for GLP-1 weight loss medications like Mounjaro and Wegovy is based on your BMI and overall health profile.
- BMI of 30 or above (obese)
- Age 18 or over
- No contraindicated conditions
- Committed to lifestyle changes
- BMI of 27 or above
- Plus type 2 diabetes
- Or high blood pressure
- Or obstructive sleep apnoea
Use our BMI calculator to check whether your BMI falls within the eligible range.
How We Built These Tools
Every calculator and guide on myweightcheck is built with accuracy and transparency in mind. Our BMI calculator uses the standard WHO formula with NHS-aligned category thresholds. Our GLP-1 projections are modelled on published clinical trial data from SURMOUNT and STEP studies.
All data stays on your device. We don't require sign-ups, and we don't store your health information on any server. Your progress tracker saves data locally in your browser using localStorage, so only you can see it.
We're based in the UK and our tools are designed specifically for UK users — supporting stone & lbs by default, referencing NHS guidelines, using NICE-approved medication data, and linking to GPhC-registered pharmacies.
Frequently Asked Questions
Common questions about BMI, weight loss medications, and our free UK tools.
Can I take Mounjaro if I'm on warfarin?
Mounjaro is not absolutely contraindicated with warfarin, but it requires careful management. Mounjaro slows stomach emptying, which may affect how your body absorbs warfarin. You must inform your prescriber and anticoagulation clinic before starting. Your INR will likely need more frequent monitoring, and your warfarin dose may need adjustment. Never start Mounjaro without this coordination.
Will Mounjaro change my INR levels?
Mounjaro may affect warfarin absorption due to delayed gastric emptying, potentially altering your INR. However, this doesn't happen to everyone, and INR changes depend on individual factors. Your anticoagulation clinic can manage this through more frequent INR blood tests and dose adjustments if needed. Regular monitoring is key.
Is Wegovy safer than Mounjaro for warfarin users?
Both Wegovy and Mounjaro slow gastric emptying and may affect warfarin absorption similarly. Neither is inherently "safer" — the choice depends on your overall health, your prescriber's experience, and your anticoagulation clinic's confidence in monitoring. Both require close coordination and regular INR checks.
What should I tell my GP before starting Mounjaro?
Tell your GP you take warfarin and any other medicines. Mention your target INR range and how stable it usually is. Ask your GP to contact your anticoagulation clinic before prescribing Mounjaro. Ensure your weight management prescriber and anticoagulation clinic can communicate with each other throughout treatment.
Will my warfarin dose need to change if I start Mounjaro?
Your warfarin dose may need adjustment once you start Mounjaro, depending on how your INR responds. This is why frequent INR monitoring (blood tests) is essential when starting. Your anticoagulation clinic will guide any dose changes based on your results. Never adjust warfarin yourself.
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