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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How Much Weight Loss With Mounjaro vs Wegovy?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections licensed in the UK, but they work differently and clinical trial results vary. On average, users lose 15–21% of body weight on Mounjaro and around 15% on Wegovy, but individual results depend on your dose, lifestyle changes, and how your body responds. This guide compares the clinical evidence and explains what determines your potential weight loss with each medication.
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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 Data: SURMOUNT vs STEP
Mounjaro (tirzepatide) was tested in the SURMOUNT-1 clinical trial, which measured weight loss in adults over 72 weeks. Results showed an average weight loss of approximately 15% at the 5mg dose, 19.5% at 10mg, and 20.9% at the maximum 15mg dose. Wegovy (semaglutide) was tested in the STEP-1 trial, which showed an average weight loss of around 14.9% at the maximum 2.4mg dose over 68 weeks. The key difference is that Mounjaro is a dual GIP and GLP-1 receptor agonist—it activates two hormone pathways—whereas Wegovy is a GLP-1 agonist only. This dual mechanism may explain why some clinical trial participants on Mounjaro achieved slightly higher average weight loss, though individual outcomes vary significantly. Both medications achieved these results alongside a reduced-calorie diet and increased physical activity, which are essential components of successful weight loss.
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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
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
What Affects Your Personal Weight Loss?
Clinical trial averages don't predict your individual result—many factors influence how much weight you'll lose. Your starting weight, age, genetics, how consistently you follow dietary guidance, physical activity level, and which dose your GP or prescriber recommends all play a role. Some people respond better to Mounjaro's dual mechanism, while others achieve their goals on Wegovy. Your prescriber will assess your medical history, BMI, and any weight-related health conditions to recommend the most suitable option. Use our free GLP-1 weight loss calculator to estimate potential outcomes at different doses, though this should not replace professional medical advice. Remember that losing even 5–10% of your body weight can deliver meaningful health benefits, regardless of which medication you take.
- 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.
Will I definitely lose weight on Mounjaro or Wegovy?
No—while clinical trials showed average weight loss, individual results vary. Some people lose more, others less. Weight loss depends on your starting weight, age, how well you follow dietary guidance, physical activity, genetics, and your dose. Both medications work best alongside a reduced-calorie diet and lifestyle changes, not as a standalone solution.
How quickly will I lose weight on these injections?
Weight loss typically becomes visible after 4–8 weeks, with more significant results after 12–16 weeks. The maximum dose is reached gradually (over several months) to minimise side effects. Safe, sustainable weight loss is around 0.5–1kg per week. Faster loss can occur initially but may include water loss and muscle loss.
Is Mounjaro more effective than Wegovy?
Clinical trials suggest Mounjaro may produce slightly higher average weight loss due to its dual GIP+GLP-1 mechanism versus Wegovy's single GLP-1 mechanism. However, the difference is modest, and individual responses vary considerably. Your prescriber will recommend based on your personal health profile, not trial averages.
What happens to weight loss if I stop taking these injections?
Weight often returns gradually if you stop the medication without maintaining dietary and lifestyle changes. These are long-term management tools. Your prescriber can discuss whether ongoing treatment is appropriate for you and how to maintain results if you discontinue.
How do I check if I'm eligible for Mounjaro or Wegovy in the UK?
Both are available for adults with a BMI of 30 or above, or 27–29.9 with weight-related health conditions (diabetes, high blood pressure, heart disease). You can calculate your BMI using our free BMI calculator, but eligibility requires clinical assessment by a GP or licensed 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.