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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Is Mounjaro Effective? How It Compares to Wegovy
Yes, clinical trials show Mounjaro (tirzepatide) is effective for weight loss in UK adults — but whether it's the right choice depends on your individual circumstances. Both Mounjaro and Wegovy are prescription-only GLP-1-based weight loss injections licensed in the UK, and both have strong evidence behind them. The key difference is their mechanism: Mounjaro uses a dual GIP and GLP-1 approach, while Wegovy uses GLP-1 alone. This guide compares their effectiveness, how they work, and what prescribers consider when choosing between them for you.
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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.
Mounjaro vs Wegovy: Clinical Trial Evidence
Both medications have been rigorously tested in large clinical trials, and both show genuine effectiveness for weight loss when combined with a reduced-calorie diet and increased physical activity.
Mounjaro (SURMOUNT-1 trial): At the maximum dose of 15 mg once weekly, participants lost an average of 20.9% of their body weight over 72 weeks. At 10 mg, average loss was 19.5%. At the starting dose of 5 mg, average loss was approximately 15%.
Wegovy (STEP-1 trial): At the maximum dose of 2.4 mg once weekly, participants lost an average of 14.9% of their body weight over 68 weeks.
These are clinical trial averages — individual results vary significantly based on adherence, diet, exercise, genetics, and other factors. Neither medication assures specific weight loss amounts. The apparent difference in average outcomes reflects both Mounjaro's dual-mechanism approach and the fact that trials used different timeframes and participant populations.
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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
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How They Work Differently: Mechanism Matters
Mounjaro's dual mechanism: Activates both GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 receptors. This may provide a stronger combined effect on appetite suppression and metabolism. The dual approach is relatively new — Mounjaro was the first dual GIP/GLP-1 medication approved for weight management.
Wegovy's single mechanism: Activates GLP-1 receptors only, which is a well-established and proven pathway. Wegovy has longer real-world use data, and some patients prefer a single-mechanism option.
In practice, which mechanism works better depends on your individual biology — there is no universal superiority. Your GP or prescriber will assess which suits your health profile, medical history, and any other medications you take. Use our free GLP-1 weight loss calculator to explore projected outcomes at different doses.
- 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.
Is Mounjaro more effective than Wegovy?
Mounjaro showed higher average weight loss in its trial (20.9% vs 14.9%), but trials had different designs and timeframes. Both are effective. "More effective" varies by individual — some respond better to Mounjaro's dual mechanism, others to Wegovy's established GLP-1 pathway. Your prescriber assesses which suits your specific situation.
How do Mounjaro doses compare to Wegovy doses?
Mounjaro ranges from 2.5 mg to 15 mg weekly; Wegovy ranges from 0.25 mg to 2.4 mg weekly. These are not directly equivalent — they're different medications with different potencies. Approximate rough equivalence: Mounjaro 15 mg ≈ Wegovy 2.4 mg, but any dose switch must be managed by your prescriber.
Can I switch from Wegovy to Mounjaro in the UK?
Yes, switching is possible under medical supervision. You cannot simply swap doses — your prescriber must assess your response to Wegovy, your current health status, and whether Mounjaro is appropriate for you. Switching typically involves stopping Wegovy and starting Mounjaro at a lower dose, with careful monitoring.
Are the side effects of Mounjaro and Wegovy the same?
Both commonly cause nausea, diarrhoea, constipation, and stomach pain — especially during dose increases. Side effect profiles are similar because both target appetite-regulating pathways. Individual tolerance varies. Mounjaro may cause slightly different patterns due to its dual mechanism, but direct comparisons show broadly similar tolerability.
How do I know which one to ask my GP about?
Both are NHS-approved through specialist weight management services, though regional availability varies. If you're accessing privately, a licensed prescriber will assess your medical history, current medications, weight loss goals, and any contraindications. Neither is "better" — the right choice is the one suitable for your individual circumstances.
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.