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
Free tracker included with every download this week
Can You Take Mounjaro With Warfarin?
If you take warfarin and are considering Mounjaro (tirzepatide), it's important to understand how these medicines interact. Mounjaro is a prescription-only weight management injection licensed in the UK, and warfarin is a blood thinner used to prevent clots. Both medications work in different ways, but there are important safety considerations when using them together. Always inform your prescriber about all medicines you take, including warfarin, before starting Mounjaro. Your GP or a licensed prescriber will assess whether it's safe for you to use both medications.
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Calculate Your BMI Instantly
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 and Warfarin Interact
Mounjaro (tirzepatide) does not have a direct pharmacological interaction with warfarin, meaning the two drugs don't interfere with each other's primary mechanisms of action. However, there are important clinical considerations:
- Weight loss effects: Mounjaro helps users lose weight by reducing appetite and supporting lower calorie intake. Significant weight loss can affect how warfarin works in your body, potentially changing how much you need
- Gastrointestinal effects: Mounjaro commonly causes nausea, vomiting, and diarrhoea — these digestive issues can affect nutrient absorption, including vitamin K, which warfarin effectiveness depends on
- Dose stability: If you're losing weight on Mounjaro, your warfarin dose may need adjustment. Your INR (international normalised ratio) will require more frequent monitoring
- Liver function: Both medications are processed by the liver; your prescriber may want to monitor liver function more carefully
The key concern is not a direct drug interaction, but rather the secondary effects of weight loss and digestive symptoms on warfarin's effectiveness.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
How It Works
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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
What You Must Tell Your Prescriber
Before starting Mounjaro, always declare to your prescriber or GP that you take warfarin. This is essential because:
- Your prescriber needs to assess whether Mounjaro is safe for you personally
- You may need more frequent INR testing (the blood test that monitors warfarin effectiveness) while starting Mounjaro
- Your warfarin dose may need adjustment once weight loss begins
- Your GP should be informed by the prescriber so both healthcare teams can coordinate care
Do not start Mounjaro without medical assessment. If you're already taking Mounjaro and thinking of starting warfarin, inform your doctor about your weight management medication. Use our free BMI calculator above to track your starting weight, and monitor changes as you progress — this helps your GP assess warfarin dose adjustments accurately.
- 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.
Will Mounjaro stop warfarin from working?
No — Mounjaro doesn't directly stop warfarin from working. However, weight loss and digestive side effects can indirectly affect warfarin's effectiveness. Significant weight loss changes how much warfarin your body needs, so your INR levels may change. More frequent blood tests are essential to keep your warfarin dose correct.
Do I need more frequent INR testing on Mounjaro?
Yes, likely. When starting Mounjaro, your prescriber and GP should arrange more frequent INR checks (possibly weekly or fortnightly initially) to monitor how warfarin is working. Once your weight stabilises on Mounjaro, INR testing frequency may return to normal. Always follow your GP's guidance on testing schedule.
Can Mounjaro's side effects affect warfarin?
Yes, possibly. Mounjaro commonly causes nausea, vomiting, and diarrhoea. Diarrhoea can affect vitamin K absorption, which warfarin depends on. If you experience severe digestive symptoms, contact your GP — they may need to adjust your warfarin dose or investigate other causes.
Is Mounjaro safe if I take warfarin?
Mounjaro may be safe with warfarin, but it requires careful monitoring. A prescriber must assess your individual health before recommending it. If approved, you'll need more frequent INR tests initially and possible warfarin dose adjustments. Never start Mounjaro without declaring your warfarin use.
What should I do if my weight loss is very rapid on Mounjaro?
Tell your GP immediately. gradual weight management on Mounjaro may require faster warfarin dose adjustments to maintain safe INR levels. Your GP may increase INR testing frequency. Keep detailed records of your weight changes using our free progress tracker to help your healthcare team make informed decisions.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.