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 Wegovy — What's the Difference?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections licensed in the UK, but they work differently and have distinct profiles. While they share similar once-weekly injection formats and eligibility criteria, the key differences lie in their mechanism, dose ranges, and clinical trial results. Understanding these differences helps you have an informed conversation with your prescriber about which medication might suit your individual health needs. This guide compares both medications factually to help you understand your options.
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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 They Work — Mechanism and Clinical Evidence
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two hormone pathways simultaneously. The SURMOUNT-1 clinical trial showed participants lost an average of 15–21% of their body weight depending on dose, with the maximum dose (15 mg once weekly) achieving approximately 20.9% average weight loss over 72 weeks alongside reduced-calorie diet and physical activity.
Wegovy (semaglutide) is a GLP-1 receptor agonist only, activating a single hormone pathway. The STEP-1 clinical trial demonstrated average weight loss of approximately 14.9% body weight at the maximum dose (2.4 mg once weekly) over 68 weeks with the same lifestyle support. Wegovy's maximum dose (2.4 mg) is considerably lower than Mounjaro's (15 mg), reflecting the different potency of their mechanisms.
Both medications slow how quickly food leaves the stomach and reduce appetite signals to the brain. Individual results vary significantly — these are clinical trial averages, not assured outcomes.
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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.
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Doses, Eligibility, and Prescriber Decision-Making
Mounjaro escalates from 2.5 mg up to 15 mg over approximately 20 weeks, while Wegovy starts at 0.25 mg and reaches 2.4 mg over approximately 16 weeks. Both require a minimum of 4 weeks at each dose before increasing. For UK eligibility, both medications are prescribed to adults with BMI 30 kg/m² or above, or BMI 27–29.9 kg/m² with weight-related health conditions such as type 2 diabetes, high blood pressure, or cardiovascular disease. South Asian, Chinese, Black African, and Black Caribbean adults may be eligible at lower BMI thresholds due to higher metabolic risk.
Your GP or prescriber will assess which medication suits you based on your personal health profile, medical history, contraindications, and treatment goals. Neither medication is inherently "better" — suitability is individual. Both require a UK-registered prescription and clinical assessment before dispensing.
- 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.
Which causes more weight loss, Mounjaro or Wegovy?
Clinical trials suggest Mounjaro produced slightly higher average weight loss (15–21%) compared to Wegovy (14.9%), but individual results vary significantly. Some people respond better to one mechanism than the other. Your prescriber will help determine which is appropriate based on your health profile, not purely on trial averages.
Can I switch from Wegovy to Mounjaro if it's not working?
Yes, switching is possible and managed by your prescriber. Approximate dose equivalence exists (Wegovy 2.4 mg roughly equals Mounjaro 10 mg), but any switch requires clinical guidance and must be planned carefully to avoid gaps in treatment.
Do Mounjaro and Wegovy have different side effects?
Both share common side effects (nausea, diarrhoea, vomiting, constipation, stomach pain) because they activate overlapping appetite-regulating pathways. Mounjaro's dual mechanism may cause slightly different gastrointestinal experiences in some people, but side effect profiles are broadly similar. Always discuss potential side effects with your prescriber.
How do I access Mounjaro or Wegovy on the NHS in the UK?
Both are available through NHS specialist tier 3 weight management services, usually requiring a GP referral. Availability and waiting times vary significantly by region and ICB. Private access is available through online clinical assessments with UK-licensed prescribers, dispensed by GPhC-registered pharmacies.
Which injection is easier to use — Mounjaro KwikPen or Wegovy pen?
Mounjaro uses the KwikPen device (pre-filled, four doses per pen) while Wegovy uses a similar pre-filled auto-injection pen. Both are once-weekly, user-friendly designs. The practical difference is minimal — your prescriber will advise on technique and storage. Some people find one more intuitive than the other, but both are designed for self-injection.
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