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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Expected Weight Loss On Mounjaro vs Wegovy
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections licensed in the UK, but they work via different mechanisms and showed different average weight loss results in clinical trials. This guide compares expected outcomes, helps you understand the trial data, and explains why individual results vary significantly. Both require clinical assessment before prescription through NHS specialist services or private online prescribing.
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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 Results — What the Evidence Shows
Mounjaro (tirzepatide) — based on SURMOUNT-1 trial data over 72 weeks:
- 5 mg dose: approximately 15% average body weight loss
- 10 mg dose: approximately 19.5% average body weight loss
- 15 mg dose (maximum): approximately 20.9% average body weight loss
Wegovy (semaglutide) — based on STEP-1 trial data over 68 weeks:
- 2.4 mg dose (maximum): approximately 14.9% average body weight loss
At equivalent doses, Mounjaro showed slightly higher average weight loss than Wegovy. However, these are trial averages alongside a reduced-calorie diet and increased physical activity — individual results vary considerably. Some people lose more, some less. Neither result is assured for any individual person.
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
How They Work Differently — Mechanism Matters
Mounjaro is a dual GIP and GLP-1 receptor agonist, meaning it activates two hormone pathways simultaneously. This dual mechanism may help explain why it showed slightly higher average weight loss in trials. Wegovy is a GLP-1 receptor agonist only, activating a single hormone pathway. Both slow gastric emptying and help reduce appetite, but Mounjaro's additional GIP pathway may provide additional metabolic support. Which medication is more suitable depends on your individual health profile, medical history, and prescriber assessment — not just trial averages.
- 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 loses more weight, Mounjaro or Wegovy?
In clinical trials, Mounjaro showed slightly higher average weight loss (approximately 20.9% at maximum dose vs 14.9% for Wegovy). However, individual results vary significantly. Your actual outcome depends on adherence, diet, activity, and metabolism — not the medication alone. A prescriber assesses which is suitable for you.
Can I use the GLP-1 weight loss calculator for both medications?
Yes. Our free weight loss projection calculator can estimate potential outcomes with either medication at different doses. Results are based on clinical trial averages but remember your actual results may differ. Use it alongside your prescriber's guidance.
What's the difference between the SURMOUNT and STEP trials?
SURMOUNT-1 tested Mounjaro over 72 weeks; STEP-1 tested Wegovy over 68 weeks. Both included reduced-calorie diet and physical activity. Trial designs were slightly different, making direct comparison approximate only. Both are robust clinical evidence for weight loss effectiveness.
If Mounjaro shows more weight loss, should I ask for Mounjaro specifically?
No — clinical trial averages don't predict individual outcomes. Prescribers assess suitability based on your health history, other medications, contraindications, and tolerability. Either medication may be recommended. Your GP or prescriber makes this decision, not trial averages.
How quickly should I see weight loss on these injections?
Weight loss typically becomes noticeable after 4–8 weeks as doses escalate. Most significant loss occurs weeks 12–24 onwards. Results plateau around 6–12 months. Using MyWeightCheck's free progress tracker helps you log weekly changes and see trends, which is motivating alongside your prescriber's monitoring.
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.