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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Semaglutide Weight Loss in the UK — How Wegovy Compares to Mounjaro
Both semaglutide (Wegovy) and tirzepatide (Mounjaro) are prescription-only weight loss injections available in the UK, but they work differently. This guide compares their mechanisms, clinical trial results, side effects, and UK eligibility to help you understand the differences. Both medications require a clinical assessment before prescription, and individual results vary significantly. Our GLP-1 weight loss calculator can help estimate potential outcomes at different doses.
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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 Semaglutide and Tirzepatide Work Differently
Semaglutide (Wegovy) is a GLP-1 receptor agonist — it activates one natural hormone pathway to reduce appetite and slow gastric emptying. Tirzepatide (Mounjaro) is a dual GIP and GLP-1 agonist, activating two pathways simultaneously. This dual mechanism may explain why clinical trials showed slightly higher average weight loss with Mounjaro (approximately 20.9% body weight at 15mg over 72 weeks) compared to semaglutide (approximately 14.9% body weight at 2.4mg over 68 weeks). However, both are effective, and individual response varies widely. Neither medication is insulin — both work by modifying appetite and energy regulation.
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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
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UK Eligibility, Access, and What Suits You
Both medications are available on the NHS through specialist tier 3 weight management services (long waiting lists in many regions) and privately through UK-licensed online prescribers following clinical assessment. Eligibility is the same: BMI 30 or above, or BMI 27–29.9 with weight-related health conditions like type 2 diabetes, high blood pressure, or sleep apnoea. For South Asian, Chinese, Black African, or Black Caribbean adults, lower thresholds apply. Your GP or prescriber will assess which is more suitable based on your medical history, other medications, and personal preference. Use our BMI calculator to check where you stand, and discuss both options with a healthcare professional.
- 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 semaglutide the same as Wegovy and Ozempic?
Semaglutide is the active ingredient in both Wegovy and Ozempic. Wegovy is licensed for weight management in the UK at up to 2.4mg weekly. Ozempic is licensed for type 2 diabetes at lower doses. Using Ozempic for weight loss is off-label in the UK. Wegovy is the licensed weight management version.
Which causes fewer side effects — semaglutide or tirzepatide?
Both have similar common side effects: nausea, diarrhoea, constipation, and stomach pain. These typically reduce over time as your body adjusts. Individual tolerance varies. Clinical trials suggest comparable side effect profiles. Your prescriber can advise based on your health history.
How much weight can I expect to lose on semaglutide?
Clinical trials showed average weight loss of approximately 14.9% of body weight at the maximum dose (2.4mg) over 68 weeks, alongside diet and exercise. Individual results vary significantly — some people lose more, some less. Results depend on dose, adherence, lifestyle changes, and individual metabolism.
Can I switch between semaglutide and tirzepatide?
Switching between medications should only be managed by your prescriber. Approximate dose equivalences exist (5mg Mounjaro ≈ 0.5mg Wegovy, 10mg ≈ 1.7mg, 15mg ≈ 2.4mg), but conversions are not official. Any switch requires clinical guidance and monitoring.
How do I get semaglutide weight loss injections on the NHS in the UK?
Ask your GP for referral to a tier 3 specialist weight management service. NHS availability varies significantly by region. Waiting lists can be lengthy. Alternatively, prescription weight loss treatment is available privately through UK-licensed online prescribers following clinical assessment.
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.