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
Free tracker included with every download this week
Does Mounjaro Cause Muscle Loss?
Mounjaro (tirzepatide) may lead to some muscle loss during weight loss, but this is not unique to this medication — it's a natural consequence of losing weight quickly. The good news is that muscle loss risk can be significantly reduced through strength training and adequate protein intake. Understanding how Mounjaro affects body composition helps you protect lean muscle mass whilst achieving your weight loss goals.
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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.
How Weight Loss Affects Muscle Mass
When you lose weight rapidly, your body breaks down both fat and muscle for energy. This is called lean mass loss and it happens with any significant weight loss — whether through diet alone, exercise, or prescription medications like Mounjaro (tirzepatide). In clinical trials, people taking Mounjaro lost substantial amounts of weight: the SURMOUNT-1 trial showed participants on the maximum 15 mg dose lost approximately 20.9% of their body weight over 72 weeks. However, the trial data did not specifically isolate muscle loss as a separate measurement. Research on similar GLP-1 weekly weight loss injections suggests that roughly 20–clinically studied weight loss may come from muscle rather than fat — which means 70–80% is fat loss, a favourable ratio. This risk is higher when weight loss is very rapid (more than 1 kg per week) or when dietary protein is inadequate.
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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.
Protecting Muscle Mass During Mounjaro Treatment
You can minimise muscle loss whilst taking Mounjaro by following these evidence-based strategies. Strength training is the most effective protection — resistance exercise signals your body to preserve muscle even during a calorie deficit. Aim for 2–3 sessions per week of weight training or bodyweight resistance work. Adequate protein intake is also critical: nutritionists recommend 1.2–2.0 g of protein per kilogramme of body weight daily during weight loss, compared to 0.8 g/kg for maintenance. Finally, avoid extremely gradual weight management — the NHS recommends losing 0.5–1 kg per week for sustainable results that preserve muscle. Pair Mounjaro with these lifestyle strategies rather than relying on the medication alone. Your GP or a registered dietitian can provide personalised guidance on protein needs and exercise programming for your specific situation.
- 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?
No — muscle loss during weight loss is not inevitable. Clinical research shows that strength training and adequate protein intake can preserve most of your lean mass. However, some muscle loss is common with any gradual weight management, including weight loss achieved with a weekly weight loss injection like Mounjaro.
How much muscle do people typically lose on Mounjaro?
Research on similar GLP-1 medications suggests approximately 20–30% of total weight loss may be from muscle, meaning 70–80% is fat loss. Individual variation is significant and depends on exercise habits, protein intake, and how quickly you lose weight. Use our free GLP-1 weight loss calculator to estimate your potential weight loss, then discuss muscle preservation strategies with your GP or dietitian.
What's the best way to prevent muscle loss on Mounjaro?
Combine strength training (2–3 times weekly) with adequate protein (1.2–2.0 g per kg body weight daily) and avoid losing weight too quickly. The NHS recommends 0.5–1 kg per week as a safe rate. These strategies work with any weight loss approach, including prescription medications like tirzepatide.
Should I take protein supplements whilst on Mounjaro?
Not necessarily — whole food protein sources (meat, fish, eggs, dairy, beans, nuts) are usually sufficient if you plan meals carefully. Supplements can help if you struggle to eat enough through food alone, particularly if Mounjaro causes nausea. Discuss your individual needs with a registered dietitian.
Is Mounjaro vs Wegovy different for muscle loss?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections that may contribute to muscle loss during gradual weight management. The protective strategies — strength training and adequate protein — apply to both medications equally. A prescriber can advise which suits your individual health situation.
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.