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- 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 Weight Loss: Before and After Results vs Wegovy
When considering tirzepatide (Mounjaro) for weight management, you've likely seen before-and-after transformations online — but what do clinical trials actually show? Mounjaro and Wegovy are both prescription weight loss injections available in the UK, but they work differently and produce different average results. This guide compares the evidence from clinical trials, explains the mechanism differences between these weekly weight loss injections, and helps you understand what a prescriber will assess when deciding which might be suitable 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.
Clinical Trial Results: SURMOUNT vs STEP Trial Data
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two hormone pathways. The SURMOUNT-1 trial tracked weight loss over 72 weeks at different doses:
- 5 mg dose: average weight loss ~15% of body weight
- 10 mg dose: average weight loss ~19.5% of body weight
- 15 mg dose (maximum): average weight loss ~20.9% of body weight
Wegovy (semaglutide) is a GLP-1 receptor agonist only — single mechanism. The STEP-1 trial showed average weight loss of approximately 14.9% at 68 weeks on the maximum 2.4 mg dose. Both injections require a reduced-calorie diet and increased physical activity to achieve results — the medication supports these efforts but does not work alone. Individual results vary significantly, and these are clinical trial averages, not assured outcomes.
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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 and Individual Suitability
The key difference is mechanism: Mounjaro activates both GIP and GLP-1 pathways, which may explain slightly higher average weight loss in trials. Wegovy activates GLP-1 only. However, "higher average" does not mean Wegovy won't work well for you — prescriber assessment and individual factors matter far more than trial averages. Your GP or a licensed prescriber will consider your medical history, current medications, contraindications, and preferences before deciding which medication (if either) is appropriate. Both are prescription-only and require clinical assessment. Neither assures specific weight loss. Try our GLP-1 weight loss calculator above to explore potential results, and use our BMI calculator to establish your 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.
Is Mounjaro more effective than Wegovy for weight loss?
Clinical trials suggest Mounjaro had slightly higher average weight loss (~20.9% vs ~14.9%), but both are effective. Individual results vary. What works best depends on your health profile, medications, and individual response — only a prescriber can assess suitability. "More effective" on average doesn't mean it's more effective for you.
What's the difference between Mounjaro and Wegovy mechanisms?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — activates two hormone pathways. Wegovy (semaglutide) is a GLP-1 agonist only — single pathway. The dual mechanism may contribute to Mounjaro's slightly higher trial results, but both slow stomach emptying and reduce appetite.
Can I switch from Wegovy to Mounjaro?
Some people do switch medications under prescriber guidance. Approximate dose equivalence exists (e.g., Wegovy 2.4 mg ≈ Mounjaro 10 mg), but any switch must be managed by your prescriber. Never self-switch — discuss with whoever prescribed your original medication.
Are before-and-after photos from real patients reliable?
Online before-and-after images are typically not representative of average results. Clinical trials provide more reliable data. Trial participants also followed strict diet and exercise protocols. Always compare your situation to NHS guidance and trial averages, not to individual stories online.
How long do Mounjaro and Wegovy take to show weight loss?
Both typically show noticeable weight loss within 4–12 weeks at therapeutic doses, though this varies. Doses increase gradually (4 weeks minimum at each dose), so reaching maximum dose takes 16–20 weeks. Clinical trials lasted 68–72 weeks. Patience with dose escalation is important for both tolerability and results.
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.