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 vs Wegovy: How Do They Compare?
Both Mounjaro and Wegovy are prescription-only weight management injections licensed in the UK, but they work differently and have distinct clinical profiles. Mounjaro (tirzepatide) uses a dual mechanism targeting two hormone pathways, whilst Wegovy (semaglutide) activates a single pathway. Understanding the differences between them—including dose ranges, trial data, and how they're prescribed—can help you have an informed conversation with your GP or prescriber about which might be suitable for you. This guide compares the key facts about each medication based on clinical evidence and UK prescribing guidelines.
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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.
Mechanism, Doses, and Clinical Trial Results
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist—meaning it activates two natural hormone pathways. This dual action may help regulate appetite, reduce food cravings, and support feeling fuller after smaller meals. In the SURMOUNT-1 clinical trial, participants taking the maximum 15 mg dose lost an average of approximately 20.9% of their body weight over 72 weeks, alongside a reduced-calorie diet and increased physical activity.
Wegovy (semaglutide) is a GLP-1 receptor agonist only—activating a single hormone pathway. In the STEP-1 clinical trial, participants taking the maximum 2.4 mg dose lost an average of approximately 14.9% of their body weight over 68 weeks, also with diet and exercise support.
- Mounjaro: Starting dose 2.5 mg, escalating to maximum 15 mg once weekly
- Wegovy: Starting dose 0.25 mg, escalating to maximum 2.4 mg once weekly
Both medications have a once-weekly injection schedule. It's important to note that clinical trial averages vary significantly between individuals—results depend on adherence, lifestyle changes, and individual metabolism.
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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.
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Eligibility, Side Effects, and Which One Might Suit You
Both medications have the same UK eligibility criteria: adults with a BMI of 30 or above, or 27–29.9 with weight-related health conditions (such as type 2 diabetes, high blood pressure, or cardiovascular disease). South Asian, Chinese, Black African, and Black Caribbean adults may be eligible at lower BMI thresholds.
Both share similar common side effects—nausea, diarrhoea, constipation, and stomach pain are very common, particularly when starting or increasing doses. Which medication is prescribed depends on individual factors: your GP or licensed prescriber will assess your medical history, current medications, and personal circumstances. Neither is inherently "better"—suitability is individual. Use our free GLP-1 weight loss calculator to explore potential outcomes at different doses, and our BMI calculator to understand your current health context before your consultation.
- 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.
Is Mounjaro more effective than Wegovy?
Clinical trials show Mounjaro achieved higher average weight loss (around 21% vs 15%), but this is trial data—individual results vary significantly. Effectiveness depends on adherence, lifestyle changes, and how your body responds. Your prescriber will recommend based on your personal health profile, not trial averages.
Can I switch from Wegovy to Mounjaro?
Switching between medications is possible but must be managed by your prescriber. There is no official conversion chart—your GP will assess the appropriate Mounjaro dose based on your individual response and health. Never adjust doses yourself.
Which has fewer side effects, Mounjaro or Wegovy?
Both share similar common side effects. Side effect severity and duration vary between individuals. Starting at the lowest dose and gradually escalating helps minimise nausea and digestive issues. Your prescriber will monitor you and adjust if needed.
Do I need a prescription for both Mounjaro and Wegovy in the UK?
Yes, both are prescription-only medicines. Access is through NHS specialist weight management services (with possible waiting lists) or private online prescribing following a clinical assessment by a UK-licensed prescriber. You cannot buy them without a valid prescription.
How do I know which one is right for me?
This is a decision for your GP or prescriber to make after assessing your medical history, current medications, BMI, and weight-related health conditions. Both are evidence-based options. Your prescriber will explain which suits your circumstances and monitor your progress once started.
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.