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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Weight Loss Injections in the UK: Wegovy vs Mounjaro
Wegovy and Mounjaro are both prescription-only weekly weight loss injections available in the UK, but they work differently and have distinct clinical profiles. This guide explains how these semaglutide and tirzepatide medications compare, what clinical trials showed, and how to access them. Both require clinical assessment before prescription. Your GP or a licensed prescriber can help determine which—if either—is suitable for your individual health needs and circumstances.
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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 Wegovy and Mounjaro Work Differently
Wegovy (semaglutide) is a GLP-1 receptor agonist, meaning it activates one natural hormone pathway that helps regulate appetite and blood sugar. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist—it activates two hormone pathways simultaneously, which is the key difference between these weekly weight loss injections.
Both medications may help reduce food cravings and support feeling fuller after smaller meals. The dual mechanism in Mounjaro may provide additional effects on fat utilisation, though individual results vary significantly. In the STEP-1 clinical trial, Wegovy users lost an average of approximately 14.9% of their body weight. In the SURMOUNT-1 trial, Mounjaro users at the maximum 15 mg dose lost an average of approximately 20.9% of body weight. These are trial averages with people also following reduced-calorie diets and increased physical activity—your own results may differ.
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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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Getting Wegovy and Mounjaro in the UK: Eligibility and Access
Both Wegovy and Mounjaro are available in the UK if you have a BMI of 30 kg/m² or above, or 27–29.9 kg/m² with a weight-related health condition such as type 2 diabetes, high blood pressure, or sleep apnoea. South Asian, Chinese, Black African, and Black Caribbean adults may be eligible at lower BMI thresholds due to higher health risks at these levels. Use the free BMI calculator on this page to check your starting point. You can access both medications through NHS specialist weight management services (with significant regional variation and waiting lists) or through private online prescribing following a clinical assessment. A prescriber will review your health history, current medications, and suitability before issuing a prescription.
- 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.
What's the main difference between Wegovy and Mounjaro?
Wegovy (semaglutide) activates the GLP-1 hormone pathway only. Mounjaro (tirzepatide) activates both GIP and GLP-1 pathways simultaneously. This dual action in Mounjaro may produce different effects, though clinical trial results show both medications support significant weight loss when used with diet and exercise changes. Individual responses vary.
Which weight loss injection is more effective?
Clinical trials showed Mounjaro users lost slightly more weight on average (approximately 20.9% at maximum dose) compared to Wegovy users (approximately 14.9%), but both are effective. However, individual results vary significantly. Suitability depends on your personal health profile, not on trial averages. Your prescriber will assess which is appropriate for you.
Can I switch from Wegovy to Mounjaro or vice versa?
Switching between medications is possible but must be managed by a prescriber. Approximate dose equivalences exist (e.g., Mounjaro 5 mg ≈ Wegovy 0.5 mg), but there is no official conversion chart. Any switch requires clinical guidance and monitoring to ensure safe transition and appropriate dosing.
How long does each weekly weight loss injection take to work?
Most people begin noticing appetite changes within 1–2 weeks of starting Wegovy or Mounjaro, though peak effects take several weeks as doses are gradually increased. Weight loss typically becomes noticeable after 4–8 weeks of consistent use alongside dietary changes. Results depend on adherence, diet, and individual metabolism.
Are Wegovy and Mounjaro available on the NHS in my area?
Both medications are NICE-approved for NHS use in England, but availability varies significantly by region and local ICB commissioning decisions. Some areas offer both; others may offer limited access. Contact your GP or local weight management service to check what's available in your area. Waiting lists can be lengthy. Private prescribing offers faster access.
Ready to take control of your weight loss journey?
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