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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Mounjaro vs Semaglutide — What's the Difference?
Mounjaro and semaglutide are often confused because they're both prescription weight management injections available in the UK, but they work in different ways. Mounjaro (tirzepatide) is a newer dual GIP and GLP-1 receptor agonist, whilst semaglutide—marketed as Wegovy for weight management—activates only the GLP-1 pathway. Both are once-weekly injections for UK adults with a BMI of 30 or above, or 27 or above with weight-related health conditions. Understanding how they differ can help you discuss options with your GP or prescriber. Use our free BMI calculator above to check your starting point, and our GLP-1 weight loss calculator to explore potential outcomes with either medication.
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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 Mounjaro and Semaglutide Work Differently
Mounjaro (tirzepatide) activates two hormone pathways: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual mechanism may amplify appetite regulation, helping you feel fuller after smaller meals and reducing food cravings more intensely than a single-pathway medication.
Semaglutide (Wegovy) activates only the GLP-1 pathway. It signals fullness to the brain and slows gastric emptying, but operates through a single mechanism. Clinical trials showed semaglutide produced average weight loss of approximately 14.9% of body weight over 68 weeks at maximum dose.
In SURMOUNT trials, Mounjaro showed average weight loss ranging from 15% (at 5 mg) to 20.9% (at 15 mg maximum dose). Individual results vary significantly—lifestyle changes and reduced-calorie diet remain essential for both medications.
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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.
Doses, Eligibility, and UK Access
Mounjaro starts at 2.5 mg weekly and escalates to a maximum of 15 mg weekly over approximately 20 weeks. Semaglutide (Wegovy) begins at 0.25 mg and reaches a maximum of 2.4 mg weekly over around 16 weeks. Both require clinical assessment—your GP or prescriber will determine suitability based on your BMI, health conditions, and medical history. Access is available through NHS specialist tier 3 weight management services (with potential waiting lists) or private online prescribing services. Neither medication is recommended during pregnancy or if you're breastfeeding. Use the weight converter on this page to check your weight in different units when discussing eligibility with your healthcare provider.
- 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.
Is Mounjaro the same as semaglutide?
No. Mounjaro (tirzepatide) and semaglutide (Wegovy) are different medications. Both are injectable weight management drugs, but Mounjaro is a dual GIP+GLP-1 agonist, whilst semaglutide is a GLP-1 agonist only. Mounjaro may produce slightly greater average weight loss in clinical trials, but results vary individually.
Which is better, Mounjaro or semaglutide?
Neither is objectively "better"—suitability depends on individual health, preferences, and prescriber assessment. Mounjaro shows marginally higher average weight loss in trials, but both are effective. Your GP or prescriber will advise which is more appropriate for your circumstances. Individual results vary.
Why do people confuse Mounjaro with semaglutide?
Both are once-weekly injectable GLP-1-related medications for weight management, often called "slimming injections" colloquially. Media coverage sometimes groups them together. However, they have different mechanisms, doses, and manufacturers—Mounjaro is made by Eli Lilly, semaglutide (Wegovy) by Novo Nordisk.
Can I switch between Mounjaro and semaglutide?
Potentially, but only under prescriber guidance. There is no official conversion chart—any switch requires medical supervision to assess your current response and adjust doses safely. Approximate equivalences exist (Mounjaro 10 mg ≈ Wegovy 1.7 mg), but individual circumstances vary.
Are both medications available on the NHS in the UK?
Both Mounjaro and Wegovy (semaglutide) are approved for NHS use, but access is through specialist tier 3 weight management services with criteria and waiting lists varying by region. Private prescription is available through UK-licensed prescribers without NHS referral. Your GP can discuss local availability.
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.