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 You Need to Know
Mounjaro (tirzepatide) is an effective weight loss injection, but gradual weight management can include some muscle loss alongside fat loss. Understanding how this happens, and what you can do about it, helps you preserve lean muscle whilst losing weight safely. This guide explains the mechanisms, prevention strategies, and how to support your health during treatment with this prescription weight loss injection.
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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.
Why Muscle Loss Happens on Mounjaro
When you lose weight rapidly—as can happen with a weekly weight loss injection like Mounjaro—your body breaks down both fat and muscle tissue for energy. This occurs because:
- Calorie deficit: Mounjaro reduces appetite, helping you eat less. A significant calorie deficit forces your body to find alternative energy sources, including muscle.
- Protein prioritisation: Without adequate protein intake, your body preferentially breaks down muscle tissue rather than preserving it.
- Inactivity: If physical activity decreases, muscles are used less and are more likely to be broken down.
- gradual weight management: The faster you lose weight, the higher the proportion of that loss that may be muscle rather than fat.
Clinical data from weight loss studies shows that without resistance exercise and adequate protein, muscle loss can account for 20–30% of total weight loss. This is why NHS guidance emphasises combining medication with lifestyle changes.
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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.
How to Minimise Muscle Loss on Mounjaro
Resistance training: Strength exercise is the most effective way to preserve muscle. Aim for at least 2–3 sessions per week of weight training or bodyweight exercises. This signals to your body that muscle is needed and worth preserving.
Adequate protein: Eat sufficient protein at each meal. Current guidance suggests 1.6–2.2g per kg of body weight daily during weight loss. This provides the amino acids your muscles need to avoid breakdown.
Gradual weight loss: Slower, steadier weight loss (0.5–1kg per week) preserves more muscle than very rapid loss. Your prescriber can adjust your Mounjaro dose or eating plan to support this pace. Use our GLP-1 weight loss calculator to track realistic expectations.
- 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 everyone lose muscle on Mounjaro?
Not necessarily. Muscle loss is most significant when weight loss is very rapid and combined with low protein intake and little physical activity. People who maintain resistance exercise and adequate protein can minimise muscle loss substantially. Individual results vary based on age, fitness level, and lifestyle choices.
How much muscle loss should I expect?
Without intervention, muscle may represent 20–30% of total weight loss. With resistance training and good protein intake, this can be reduced to 5–10%. For example, if losing 20kg with these measures, you might preserve 18–19kg as fat loss and only lose 1–2kg of muscle.
Is muscle loss permanent after stopping Mounjaro?
No. Muscle is metabolically active tissue that can be rebuilt through resistance training and adequate nutrition after weight loss treatment ends. Many people regain muscle relatively quickly when they resume strength training and maintain good protein intake.
How much protein do I need on Mounjaro?
Aim for 1.6–2.2g per kg of your current body weight daily. For example, at 80kg, this means 128–176g protein daily, spread across meals. Good sources include chicken, fish, eggs, dairy, legumes, and protein powder. Your GP or a registered dietitian can provide personalised guidance.
Can I use a BMI calculator to track whether I'm losing muscle?
BMI calculators measure overall weight but cannot distinguish between fat and muscle loss—this is an important limitation of BMI. Your GP can assess body composition changes through measurements like waist circumference or, if available, body composition analysis. Use our free BMI calculator to track overall progress, but discuss muscle preservation strategies with your healthcare team.
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.