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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Muscle Loss With Mounjaro — What UK Adults Need to Know
When taking Mounjaro (tirzepatide) for weight management, many people worry about losing muscle alongside fat. While Mounjaro helps you lose weight by reducing appetite and supporting fullness, it's important to understand how this affects your muscle mass and what you can do to preserve it. This guide explains muscle loss on Mounjaro in a UK context, with practical strategies recommended by health professionals.
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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.
How Mounjaro Affects Muscle During Weight Loss
Mounjaro works by activating GIP and GLP-1 receptor pathways, which reduces appetite and slows gastric emptying — helping you feel fuller on less food. However, any significant weight loss, whether with or without medication, can result in some muscle loss alongside fat loss. This happens because your body uses both fat and muscle for energy when in a calorie deficit.
Clinical trials showed that adults using Mounjaro (a weekly weight loss injection) lost an average of 15–21% of their body weight over 72 weeks. The proportion of muscle lost versus fat lost depends on several factors: your current muscle mass, the speed of weight loss, your protein intake, and how much physical activity you do. Faster weight loss generally results in greater muscle loss, which is why the NHS recommends losing weight at a rate of 0.5–1 kg per week for sustainable results.
Key point: Muscle loss during weight loss is normal, but it can be minimised through resistance exercise and adequate protein intake.
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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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Protecting Your Muscle Mass on Mounjaro
To preserve lean muscle while taking Mounjaro, follow these evidence-based strategies:
- Strength training: Resistance exercise (weightlifting, bodyweight exercises) signals your body to maintain muscle. Aim for 2–3 sessions per week targeting major muscle groups.
- Adequate protein: Consume sufficient protein (approximately 1.6–2.0g per kg of body weight daily). This provides amino acids your muscles need to stay strong.
- Gradual dose increases: Ask your prescriber about mounjaro dose titration — slower progression may reduce gradual weight management and associated muscle loss.
- Consistent activity: Combine resistance training with regular movement (walking, cycling) to maintain overall fitness.
- Monitor progress: Track your progress with our free progress tracker to identify whether you're losing weight sustainably. If weight loss is very rapid (more than 1–1.5 kg per week), discuss this with your GP.
Your prescriber can also help assess whether the speed of your weight loss is healthy and sustainable for your individual circumstances.
- 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 muscle on Mounjaro?
Some muscle loss can occur during any significant weight loss, but it's not inevitable. By combining resistance training, adequate protein intake, and gradual weight loss (0.5–1 kg per week), you can minimise muscle loss. Many people using Mounjaro successfully preserve muscle whilst losing fat.
How much protein should I eat while on Mounjaro?
Aim for approximately 1.6–2.0g of protein per kilogramme of body weight daily. For a 70kg adult, this means around 112–140g of protein per day. This supports muscle maintenance during weight loss. A registered dietitian can provide personalised guidance.
Can I use the BMI calculator to track muscle changes?
BMI measures weight relative to height but doesn't distinguish between muscle and fat — it's a screening tool, not a body composition measure. Our free BMI calculator helps track overall weight change, but for muscle preservation specifically, resistance training and strength assessments are more useful indicators.
What does the NHS say about muscle loss and weight loss medication?
The NHS emphasises that weight loss medications like Mounjaro should be combined with lifestyle changes including physical activity and a balanced diet. NHS guidance recommends resistance exercise and adequate nutrition to support health during weight loss.
How is Mounjaro prescribed through the NHS in the UK?
Mounjaro is available through NHS tier 3 specialist weight management services following GP referral. A weekly weight loss injection requires ongoing clinical assessment. You can also access Mounjaro privately through UK-licensed prescribers. Your GP can discuss which route suits your circumstances.
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.