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 — What the Research Shows
If you're considering prescription weight loss treatment in the UK, you've likely come across both Mounjaro (tirzepatide) and Wegovy (semaglutide). Both are licensed for weight management and supported by clinical trial evidence, but they work differently and showed different results. Understanding the research behind each can help you and your GP or prescriber decide which might be more suitable for your individual circumstances. This guide breaks down the key trial data, mechanism differences, and practical implications for UK patients.
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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.
Clinical Trial Data — SURMOUNT vs STEP
Mounjaro (tirzepatide) was evaluated in the SURMOUNT-1 trial, which ran for 72 weeks with over 2,500 participants. The results showed average weight loss of approximately 15% of body weight at the 5 mg dose, 19.5% at 10 mg, and 20.9% at the 15 mg maximum dose — all compared to placebo. These results were achieved alongside a reduced-calorie diet and increased physical activity.
Wegovy (semaglutide) was studied in the STEP-1 trial over 68 weeks with similar participant numbers. At the maximum dose of 2.4 mg, participants lost an average of approximately 14.9% of their body weight — again, with dietary and exercise changes. Individual results in both trials varied significantly, and weight loss was not assured for all participants.
Both trials were rigorous, placebo-controlled studies published in peer-reviewed medical journals. However, direct head-to-head comparison is not straightforward because the trials differed in duration, population, and design. The key finding: both medications supported meaningful weight loss in clinical settings, but results varied between individuals.
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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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How They Work Differently — Mechanism Matters
The most significant difference is in how these two medications work at the cellular level. Wegovy is a GLP-1 receptor agonist — it activates a single natural hormone pathway (GLP-1) that helps regulate appetite, food intake, and blood sugar. Wegovy works by making you feel fuller, reducing cravings, and slowing how quickly food leaves your stomach.
Mounjaro is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways (GIP and GLP-1) simultaneously. The addition of GIP activation may provide extra benefit in appetite regulation and fat utilisation, which is why some research suggests dual-action medication might produce greater weight loss in some people. However, this does not mean Mounjaro is automatically "better" — individual response varies considerably, and the single-pathway approach of Wegovy works well for many patients.
- 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.
Which medication caused more weight loss in research trials?
Mounjaro showed slightly higher average weight loss (up to 20.9%) compared to Wegovy (14.9%) in their respective trials. However, these were separate trials with different designs, so direct comparison is not straightforward. Individual results vary significantly — some people respond better to one medication than the other.
Does the difference in mechanism mean Mounjaro is better for everyone?
No. The dual-action mechanism of Mounjaro may benefit some people, but Wegovy's single GLP-1 action is effective for many patients. Suitability depends on individual health factors, medical history, and how your body responds. Your prescriber will assess which is more appropriate for you.
Are Mounjaro and Wegovy available on the NHS in the UK?
Both are licensed for weight management in the UK. NHS availability is through specialist tier 3 weight management services and varies significantly by region. Both are also available through private prescription following an online clinical assessment with a UK-licensed prescriber.
What happens if one medication doesn't work well for me?
If you experience intolerable side effects or inadequate weight loss on one medication, a prescriber may consider switching to the other. Switches should be managed carefully under medical guidance. Some people respond better to one mechanism than another, which is why flexibility in prescribing is important.
Can I use the MyWeightCheck GLP-1 calculator to compare potential outcomes?
Yes. Our free GLP-1 weight loss calculator can help you estimate potential weight loss projections based on different doses and starting weight. You can also use our BMI calculator and weight converter to understand your current baseline and track progress throughout treatment.
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.