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 Clinical Trial vs Wegovy — Evidence Compared
Both Mounjaro and Wegovy are prescription-only weight management injections available in the UK, but they come from different clinical trials with different mechanisms. This guide compares the SURMOUNT clinical trial (Mounjaro) with the STEP trials (Wegovy), helping you understand the evidence behind each medication. Both have been assessed by UK regulators and are available through NHS specialist services or private prescribing following individual clinical assessment.
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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.
SURMOUNT vs STEP — Trial Design and Weight Loss Results
Mounjaro (tirzepatide) — SURMOUNT-1 Trial: The primary trial ran for 72 weeks and included approximately 2,500 participants. At the maximum dose of 15 mg once weekly, participants lost an average of 20.9% of their body weight. At 10 mg weekly, average weight loss was 19.5%. At the lower 5 mg dose, average loss was approximately 15%. These results were achieved alongside a reduced-calorie diet and increased physical activity.
Wegovy (semaglutide) — STEP-1 Trial: The primary trial also ran for 68 weeks with approximately 1,900 participants. At the maximum dose of 2.4 mg once weekly, participants lost an average of 14.9% of their body weight. Like Mounjaro, results were achieved alongside lifestyle changes. Individual results in both trials varied significantly between participants.
Key difference: Mounjaro trials showed slightly higher average weight loss percentages, but both medications demonstrated clinically meaningful results. Trial design and participant populations differed slightly, making direct comparison complex.
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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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Mechanism Differences — Why Mounjaro and Wegovy Work Differently
Mounjaro is a dual GIP + GLP-1 receptor agonist, meaning it activates two natural hormone pathways: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual action may contribute to appetite regulation and the weight loss results seen in the SURMOUNT trials.
Wegovy is a GLP-1 receptor agonist only, activating just the GLP-1 pathway. This single mechanism is highly effective but operates differently from Mounjaro's dual approach. Both mechanisms help reduce appetite, support feeling fuller, and slow gastric emptying. Your GP or prescriber will assess which mechanism may be more suitable for your individual health and medical history.
- 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 was the SURMOUNT clinical trial?
SURMOUNT-1 was the primary efficacy trial for Mounjaro (tirzepatide) for weight management. It ran for 72 weeks with approximately 2,500 UK and international participants. Participants received Mounjaro doses ranging from 5 mg to 15 mg once weekly alongside lifestyle changes. Results showed average weight loss of 15–20.9% depending on dose, establishing Mounjaro's effectiveness for UK regulatory approval.
What was the STEP clinical trial?
STEP-1 was the primary efficacy trial for Wegovy (semaglutide) for weight management. The 68-week trial included approximately 1,900 participants. Participants received Wegovy up to 2.4 mg once weekly alongside a reduced-calorie diet and increased physical activity. Average weight loss was 14.9% at the maximum dose, supporting Wegovy's UK licencing for weight management.
Which shows better weight loss results — SURMOUNT or STEP?
SURMOUNT (Mounjaro) trials showed slightly higher average weight loss percentages (up to 20.9%) compared to STEP (Wegovy) at approximately 14.9%. However, both trials had different designs and participant populations. Individual results vary significantly. Your prescriber will assess which medication may be more suitable based on your health, not trial averages alone.
Are these trials relevant to UK patients?
Yes. Both SURMOUNT and STEP trials included UK and international participants. Results were reviewed by the MHRA and NICE during UK licensing approval. However, your individual response may differ from trial averages. A clinical assessment with your GP or prescriber determines whether either medication is appropriate for you personally.
Can I use MyWeightCheck tools to estimate my potential results?
Yes. Our free GLP-1 weight loss calculator can help you explore potential weight loss projections at different doses based on clinical trial data. Combined with our BMI calculator and weight converter, these tools help you understand the evidence in a UK context before discussing options with your 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.