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: UK Results and Clinical Trial Data
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections licensed in the UK, but they work differently and show different results in clinical trials. This guide compares the two medications side-by-side, looking at what the SURMOUNT and STEP trials revealed, how they differ as treatments, and how your prescriber decides which is right for you. Individual results vary significantly between people.
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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.
SURMOUNT vs STEP: Clinical Trial Results Compared
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist tested in the SURMOUNT-1 trial over 72 weeks. At the maximum dose of 15 mg weekly, participants lost an average of 20.9% of their body weight alongside a reduced-calorie diet and exercise. At 10 mg, average weight loss was 19.5%, and at the starting 5 mg dose, approximately 15%. Wegovy (semaglutide) is a GLP-1-only receptor agonist tested in the STEP-1 trial over 68 weeks. At the maximum dose of 2.4 mg weekly, participants lost an average of 14.9% of body weight with the same lifestyle support. These are averages — some people lost more, others less. The dual mechanism in Mounjaro (targeting both GIP and GLP-1 pathways) is why clinical trial results show a numerical advantage, but both medications require a tirzepatide uk prescription from a licensed prescriber and proper clinical assessment before starting.
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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 and Who Suits Which
Mechanism matters. Mounjaro activates two hormone pathways simultaneously (GIP and GLP-1), which may explain why trial participants saw slightly greater average weight loss. Wegovy activates the GLP-1 pathway only, which is still highly effective but through a single mechanism. Both slow stomach emptying and reduce appetite. Your prescriber will assess you individually — considering your medical history, other medications, previous weight loss attempts, and any specific health conditions. Neither medication is objectively "better"; suitability depends on your personal circumstances. Both require ongoing monitoring and lifestyle changes. Use our free GLP-1 weight loss calculator to project potential outcomes at different doses for either medication, and our BMI calculator to understand your current health baseline.
- 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 injection gives faster weight loss results in the UK?
Mounjaro showed numerically higher average weight loss in clinical trials (20.9% at maximum dose vs Wegovy's 14.9%), but these are population averages. Individual results vary significantly and depend on adherence, diet, exercise, genetics, and metabolism. Your prescriber will discuss which suits your circumstances.
Does Mounjaro work better than Wegovy for everyone?
No. Clinical trial data shows Mounjaro's average advantage, but many people achieve excellent results with Wegovy. Effectiveness varies person-to-person. Some people respond better to GLP-1-only treatment; others benefit from the dual GIP+GLP-1 mechanism. Your prescriber decides based on your health profile.
Can I switch from Wegovy to Mounjaro or vice versa in the UK?
Yes, but only with prescriber guidance. There's no official NHS conversion chart between the medications because they have different mechanisms and doses. Any switch must be managed by your GP or private prescriber to ensure safe transition and appropriate dosing.
Which prescription weight loss UK injection is available on the NHS?
Both Mounjaro and Wegovy are approved by NICE for NHS use through specialist tier 3 weight management services, but NHS availability varies significantly by region. Waiting lists can be very long. Both are available privately through GPhC-registered online prescribers without NHS referral.
What does the SURMOUNT trial tell us about Mounjaro UK results?
SURMOUNT-1 tested tirzepatide over 72 weeks and showed 20.9% average weight loss at 15 mg weekly, 19.5% at 10 mg, and 15% at 5 mg. Participants also followed reduced-calorie diets and exercise. Weight loss continued as doses increased, supporting the importance of dose titration during treatment.
Ready to take control of your weight loss journey?
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