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 Studies vs Wegovy — What the Clinical Trials Show
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections licensed in the UK, but they're backed by different clinical trials with different results. Understanding the SURMOUNT and STEP studies helps you and your GP or prescriber make an informed decision about which medication might suit you best. This guide compares the evidence, mechanisms, and real-world suitability for UK adults.
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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 — What the Clinical Trials Showed
Mounjaro's SURMOUNT Trial involved over 2,500 adults and tested tirzepatide at doses of 5mg, 10mg, and 15mg once weekly over 72 weeks. Results showed average weight loss of approximately 15% at 5mg, 19.5% at 10mg, and 20.9% at 15mg of body weight.
Wegovy's STEP Trial involved similar numbers and tested semaglutide doses from 0.25mg to 2.4mg once weekly over 68 weeks. Average weight loss was approximately 14.9% of body weight at the maximum dose.
Key difference: Mounjaro uses a dual GIP and GLP-1 mechanism — activating two hormone pathways. Wegovy uses a single GLP-1 mechanism only. This dual action in Mounjaro may explain the slightly higher average weight loss observed in the SURMOUNT trial, though individual results vary considerably. Both trials included reduced-calorie diet and increased physical activity alongside the medication.
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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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Mechanism Differences and Individual Suitability
The fundamental difference lies in how these medications work. Mounjaro's tirzepatide activates both GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 receptors, affecting appetite, fullness signalling, and how the body processes glucose and fat. Wegovy's semaglutide targets GLP-1 only. For some people, the dual mechanism provides better appetite control; others respond equally well to GLP-1 alone. Neither drug is universally "better" — prescriber assessment determines suitability based on your medical history, other medications, and individual response. Both are available through NHS specialist services or private prescribing following clinical assessment. Try our GLP-1 weight loss calculator to explore potential outcomes, and use our BMI calculator to check your eligibility 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.
Did the SURMOUNT trial show Mounjaro is more effective than Wegovy?
SURMOUNT showed higher average weight loss than STEP (20.9% vs 14.9%), but direct comparison is complicated because the trials differed in duration, populations, and baseline characteristics. Both are effective; individual response varies significantly.
How do the mechanisms differ between Mounjaro and Wegovy?
Mounjaro (tirzepatide) is a dual GIP+GLP-1 agonist, activating two hormone pathways. Wegovy (semaglutide) is a GLP-1 agonist only. The dual mechanism may enhance appetite regulation and fat metabolism in some people, but both work effectively for weight management.
Which medication did UK clinical guidelines recommend?
Both Mounjaro and Wegovy are NICE-approved and available in UK specialist weight management services. NICE has approved tirzepatide (Mounjaro), but neither drug is universally preferred — your prescriber assesses suitability based on your individual health.
Can I switch from Wegovy to Mounjaro or vice versa?
A prescriber can help you switch if one medication isn't suitable. There's no official established conversion between the two — approximately, 1.7mg Wegovy may be equivalent to 10mg Mounjaro, but any switch requires medical guidance and careful dose adjustment.
What was the UK KwikPen approval based on?
Mounjaro's UK KwikPen formulation was approved based on SURMOUNT trial data and additional real-world evidence. The MHRA assessed efficacy, safety, and manufacturing quality before approving it for UK prescription.
Ready to take control of your weight loss journey?
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