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 for Weight Loss: What's the Difference?
Both Mounjaro and Wegovy are prescription-only weekly weight loss injections licensed in the UK, but they work through different mechanisms. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, whilst Wegovy (semaglutide) is a GLP-1 agonist only. Clinical trials suggest Mounjaro may produce slightly greater average weight loss, but individual results vary significantly. Both require a clinical assessment before prescription and work best alongside dietary changes and increased physical activity. Understanding the differences helps you discuss options with your GP or prescriber.
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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 Wegovy Work Differently
The key difference lies in their mechanism of action. Mounjaro activates two hormone pathways — GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) — whilst Wegovy targets GLP-1 only. This dual action in Mounjaro may explain why clinical trials showed slightly higher average weight loss.
In the SURMOUNT-1 trial, Mounjaro at 15 mg produced an average weight loss of approximately 20.9% of body weight over 72 weeks. Wegovy, tested in the STEP-1 trial, achieved approximately 14.9% average weight loss over 68 weeks at the maximum dose of 2.4 mg. However, individual results vary considerably — some people lose more on Wegovy, some lose less on Mounjaro.
Both medications slow how quickly food leaves your stomach, reduce appetite, and help you feel fuller after smaller meals. Both are given as once-weekly injections using a pre-filled pen (Mounjaro's KwikPen or Wegovy's auto-injection pen).
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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, Side Effects, and Access in the UK
Dosing differences: Mounjaro starts at 2.5 mg and escalates to a maximum of 15 mg weekly, with dose increases every 4 weeks. Wegovy begins at 0.25 mg and increases to a maximum of 2.4 mg weekly over approximately 16 weeks — a slower titration.
Side effects are similar: nausea, diarrhoea, constipation, and stomach pain are common early on, typically improving with time. Both are prescription-only and require a clinical assessment.
UK access: Both medications are available through NHS specialist weight management tier 3 services (with potentially long waiting lists) or through private online prescribing following a consultation with a UK-licensed prescriber. Your GP or prescriber will assess which option suits your individual health profile. Use our GLP-1 weight loss calculator to explore potential outcomes with either medication.
- 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 is better for weight loss — Mounjaro or Wegovy?
Clinical trials suggest Mounjaro may produce slightly higher average weight loss (around 21% vs 15%), but individual results vary significantly. What works best depends on your personal health, tolerability, and how your body responds. Only a prescriber can assess which is more suitable for you after a clinical assessment. Results vary considerably between individuals.
Can I switch from Wegovy to Mounjaro?
Yes, switching is possible, but it must be managed by your prescriber. Whilst approximate dose equivalences exist (e.g., Wegovy 2.4 mg is roughly equivalent to Mounjaro 15 mg), there is no official conversion chart. Your prescriber will advise on the safest way to transition and may restart titration protocols. Any switch requires clinical supervision.
What's the difference between Ozempic and Wegovy?
Both contain semaglutide, but Ozempic is approved for type 2 diabetes (up to 2 mg dose) and is not licensed for weight loss in the UK. Wegovy is the licensed weight management version at higher doses (up to 2.4 mg). Using Ozempic for weight loss is off-label. Always use the licensed medication for your intended purpose.
Are Mounjaro and Wegovy available on the NHS?
Both are approved for NHS specialist tier 3 weight management services, but availability and waiting times vary significantly by region and local ICB. Many areas have long waiting lists. Private access is available through GPhC-registered online pharmacies following a consultation with a UK-licensed prescriber, typically costing £129–£320+ monthly depending on dose.
How long do you stay on Mounjaro or Wegovy?
Both are typically continued as long as they're working and well-tolerated. There's no fixed endpoint — treatment duration depends on your individual health goals and response. Stopping either medication may result in weight regain if dietary and lifestyle changes aren't maintained. Discuss long-term plans with your prescriber, and use our progress tracker to monitor your results over time.
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.