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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Does Mounjaro Burn Muscle? What Clinical Evidence Shows
Weight loss medications like Mounjaro (tirzepatide) and Wegovy (semaglutide) can potentially cause some muscle loss alongside fat loss, particularly if diet and exercise are not optimised. Both medications work by reducing appetite and food intake, which means your body receives fewer calories. Without adequate protein and resistance exercise, weight loss from any source—medication or diet alone—may include some lean muscle. The key difference between Mounjaro and Wegovy lies in their mechanisms and how individual bodies respond, though neither medication specifically "burns" muscle preferentially.
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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 Trial Data: Muscle Preservation Evidence
The SURMOUNT-1 clinical trial for Mounjaro (tirzepatide) measured weight loss at different doses over 72 weeks. At the maximum 15 mg dose, participants lost approximately 20.9% of body weight on average. However, the trial did not specifically report separate lean muscle vs fat mass loss—only total weight reduction was published.
The STEP-1 trial for Wegovy (semaglutide) showed approximately clinically studied weight loss at 68 weeks on 2.4 mg. Like SURMOUNT, STEP-1 focused on total weight reduction rather than body composition breakdown.
Neither trial specifically addressed muscle preservation. In practice, muscle loss during GLP-1 or dual GIP/GLP-1 therapy depends more on protein intake and resistance exercise than on the medication itself. Both Mounjaro and Wegovy reduce appetite, so eating adequate protein requires deliberate effort—this is where individual behaviour, not the medication, determines muscle retention.
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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.
Individual Suitability: Which Is Right for You?
Deciding between Mounjaro and Wegovy should be made with your prescriber, not based on muscle loss concern alone. Both medications require prescription and clinical assessment—neither is available over-the-counter or without a qualified healthcare professional's review.
If muscle preservation is important to you, discuss this with your GP or prescriber. They can advise on: adequate protein targets (typically 1.6–2.0g per kg body weight during weight loss), resistance training twice weekly, and monitoring body composition rather than weight alone using our free BMI calculator and progress tracker to track changes alongside measurements. Your prescriber may also discuss dose titration timing, which affects how quickly appetite suppression occurs and allows more gradual dietary adjustment.
- 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 Mounjaro cause me to lose muscle mass?
Mounjaro (tirzepatide) reduces appetite, which may lead to reduced calorie intake. Any weight loss—from medication or calorie restriction—can include some muscle loss if protein intake is inadequate or resistance exercise is not performed. The medication itself does not specifically target muscle; muscle preservation depends on your diet (adequate protein) and exercise routine. Discuss protein targets with your GP or a registered dietitian.
Is Wegovy better for preserving muscle than Mounjaro?
Clinical trials do not directly compare muscle preservation between Wegovy and Mounjaro. Both are GLP-1-based medications that work through appetite reduction. Muscle preservation with either medication depends primarily on maintaining adequate protein intake and performing resistance exercise, not on which medication you use. Your prescriber can advise which is more suitable for your individual circumstances.
How much protein should I eat while taking Mounjaro or Wegovy?
During weight loss, adequate protein helps preserve muscle. General guidance is 1.6–2.0g of protein per kg of body weight daily. However, GLP-1 medications reduce appetite, making high protein intake challenging. A registered dietitian can create a personalised plan. Prioritise protein at each meal—eggs, fish, lean meat, Greek yoghurt, lentils, and tofu are practical options in the UK.
What does the NHS say about muscle loss with weight loss medication?
NHS guidance emphasises that sustainable weight loss combines medication (where prescribed), reduced-calorie diet, and increased physical activity. Resistance exercise is specifically recommended to preserve muscle during weight loss. The NHS also recommends losing no more than 0.5–1 kg per week to minimise muscle loss risk. Always follow your prescriber's advice on diet and exercise alongside any medication.
Can I use the progress tracker to monitor muscle loss on Mounjaro?
Our free progress tracker helps log weight changes over time. However, tracking weight alone does not distinguish between muscle and fat loss. Consider also measuring waist circumference, taking progress photos, and noting how clothes fit. For precise body composition (muscle vs fat %), ask your GP about a DEXA scan or bioelectrical impedance analysis, though these are not routinely available on the NHS for weight management.
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