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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Wegovy vs Mounjaro: Weight Loss Injections Compared
Wegovy and Mounjaro are the two most commonly prescribed weekly weight loss injections in the UK. Both are prescription-only medicines licensed for adults with excess weight, but they work differently and have different dose ranges. This guide explains the key differences between semaglutide (Wegovy) and tirzepatide (Mounjaro), clinical trial evidence, and what to expect. Your GP or prescriber will assess which may be suitable for your individual circumstances.
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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 Wegovy and Mounjaro Work Differently
Wegovy (semaglutide) is a GLP-1 receptor agonist — it activates one hormone pathway to reduce appetite and support feeling fuller after meals. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two pathways, which may explain why clinical trials showed slightly higher average weight loss with Mounjaro.
- Wegovy: GLP-1 only; maximum dose 2.4 mg once weekly
- Mounjaro: GIP + GLP-1; maximum dose 15 mg once weekly
Both slow how quickly food leaves the stomach and may help reduce cravings. Individual response varies significantly — some people see better results with one medication over the other. Only a prescriber can assess which mechanism may suit your health profile and goals.
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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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Clinical Trial Evidence and Weight Loss Results
Wegovy (STEP trials): Average weight loss of approximately 14.9% of body weight at 68 weeks on the 2.4 mg maintenance dose.
Mounjaro (SURMOUNT trials): Average weight loss ranging from approximately 15% (5 mg dose) to 20.9% (15 mg dose) at 72 weeks.
These are average results alongside reduced-calorie diet and increased physical activity — individual outcomes vary widely. Neither medication assures specific weight loss amounts. Use our GLP-1 weight loss calculator to project potential outcomes at different doses, though actual results depend on your personal commitment to lifestyle changes, metabolism, and how your body responds to the medication.
- 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.
What is the main difference between Wegovy and Mounjaro for weight loss?
Wegovy (semaglutide) is a single GLP-1 receptor agonist, while Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist. The dual mechanism in Mounjaro may provide slightly higher average weight loss in clinical trials, but individual response varies. Your prescriber will assess which suits your health profile.
Are Wegovy and Mounjaro available on the NHS in the UK?
Both are available through NHS specialist tier 3/4 weight management services, but access is limited and varies by region. Waiting lists can be lengthy. Both are more readily available through private prescription following an online clinical assessment with a UK-licensed prescriber.
Can you switch from Wegovy to Mounjaro or vice versa?
Switching is possible but must be managed by your prescriber. There is no official conversion chart — approximate equivalences exist (e.g., Wegovy 2.4 mg ≈ Mounjaro 15 mg), but any switch requires clinical guidance and may involve a transition period.
Which injection causes fewer side effects, Wegovy or Mounjaro?
Both have similar common side effects: nausea, diarrhoea, vomiting, and constipation. Side effect profiles are comparable. Individual tolerance varies — some people tolerate one better than the other. Your GP can discuss which may be safer given your medical history.
How do I know which weight loss injection is right for me?
Only a licensed prescriber can assess your eligibility and suitability after a clinical consultation. Factors include your BMI, health conditions, medications, medical history, and preferences. Use our free BMI calculator to check where you fall, then discuss options with your GP or a private prescriber.
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.