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 Injection Results vs Wegovy: What the Evidence Shows
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weekly weight loss injections available in the UK, but they work differently and produced different results in clinical trials. This guide compares the injection results head-to-head, explains the mechanism differences, and helps you understand what individual outcomes might look like. Use our free BMI calculator to check your starting point, and our GLP-1 weight loss calculator to see projected results with either medication.
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Calculate Your BMI Instantly
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: SURMOUNT vs STEP Data
Mounjaro (tirzepatide) showed stronger average weight loss in the SURMOUNT-1 clinical trial. At the maximum dose of 15 mg weekly, participants lost an average of 20.9% of their body weight over 72 weeks when combined with a reduced-calorie diet and increased physical activity. At 10 mg, the average was 19.5%. At the starting 5 mg dose, participants lost approximately 15%.
- SURMOUNT-1 context: 2,539 adults with BMI ≥30 or BMI 27–29.9 with weight-related conditions
- Mounjaro mechanism: Dual GIP and GLP-1 receptor agonist — targets two hormone pathways simultaneously
Wegovy (semaglutide) demonstrated solid results in the STEP-1 trial. At the maximum dose of 2.4 mg weekly, participants achieved an average weight loss of 14.9% of body weight over 68 weeks alongside lifestyle changes. This is clinically meaningful but typically lower than Mounjaro's figures at equivalent time points.
- STEP-1 context: 1,961 adults with BMI ≥30 or overweight with comorbidities
- Wegovy mechanism: Single GLP-1 receptor agonist — targets one hormone pathway
Key context: These are average results from controlled trials. Individual results vary significantly based on adherence, diet, exercise, genetics, metabolism, and starting weight.
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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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Mechanism Differences and Individual Suitability
The fundamental difference is that Mounjaro targets two appetite and metabolic pathways (GIP and GLP-1), while Wegovy targets only GLP-1. This dual mechanism may explain why Mounjaro users, on average, achieved greater weight loss in trials. However, both medications slow gastric emptying, reduce appetite, and support satiety — the difference is one of degree, not principle.
Which medication is suitable for you depends on your individual circumstances: your BMI, any weight-related health conditions, current medications, contraindications, and your GP or prescriber's clinical assessment. Neither medication is "better" in absolute terms — they're different tools that work differently for different people. Some individuals respond excellently to Wegovy; others achieve better results with Mounjaro. Prescribers choose based on your full health picture, not trial averages. Track your progress with our free progress tracker once you start treatment.
- 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.
Does Mounjaro always produce more weight loss than Wegovy?
No. Clinical trials show Mounjaro achieved higher average weight loss, but individual results vary dramatically. Some Wegovy users lose significant weight; some Mounjaro users lose less than the trial average. Your personal outcome depends on genetics, metabolism, adherence, diet, exercise, and other factors — not the drug alone.
What is the difference between tirzepatide and semaglutide?
Tirzepatide (Mounjaro) is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways. Semaglutide (Wegovy) is a single GLP-1 receptor agonist — it activates one pathway. Both support weight loss, but the dual mechanism may allow slightly stronger average results.
Can I switch from Wegovy to Mounjaro or vice versa?
Yes, but only under medical supervision. There is no official conversion dose — prescribers typically restart titration (gradual dose increases) when switching. An approximate rough guide: Wegovy 2.4 mg is sometimes compared to Mounjaro 10–15 mg, but this varies. Always discuss with your prescriber.
Which weekly weight loss injection is available on the NHS in the UK?
Both Mounjaro and Wegovy are NICE-approved for NHS use, but access is through specialist tier 3/4 weight management services, not standard GP prescription. Availability and waiting times vary significantly by region. Private prescription is faster but costs around £150–£320/month depending on dose.
How do I know which injection will work better for me?
You don't know in advance — individual response is unpredictable. Your GP or online prescriber assesses your health, BMI, comorbidities, and current medications to recommend one option. Some prescribers start with one and switch if results are suboptimal, but this requires repeat clinical assessment and patience.
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.