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 loss injections licensed in the UK, but they work differently and have different clinical profiles. Recent news about weight loss treatments has brought these medications into focus. Understanding the key differences — including how they work, trial outcomes, and dosing — helps you and your GP or prescriber make an informed decision about which might suit your situation. Both require clinical assessment and are used alongside lifestyle changes.
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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 — Clinical Trial Data
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways to regulate appetite and blood sugar. In the SURMOUNT-1 clinical trial, Mounjaro showed average weight loss of approximately 15% at 5mg, 19.5% at 10mg, and 20.9% at 15mg over 72 weeks. Maximum dose is 15mg once weekly.
Wegovy (semaglutide) is a GLP-1 receptor agonist only, activating a single hormone pathway. The STEP-1 clinical trial showed average weight loss of approximately 14.9% at the maximum 2.4mg dose over 68 weeks. Both medications are used alongside a reduced-calorie diet and increased physical activity — results vary significantly between individuals.
- Mounjaro: dual mechanism (GIP + GLP-1)
- Wegovy: single mechanism (GLP-1 only)
- Mounjaro doses: 2.5mg to 15mg weekly
- Wegovy doses: 0.25mg to 2.4mg weekly
Our free GLP-1 weight loss calculator can help you explore projected outcomes at different doses for either medication.
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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.
Which Medication Is Right for You?
The choice between Mounjaro and Wegovy is not about one being universally "better" — it's about individual suitability. Your prescriber will assess your medical history, current medications, and health goals to recommend the most appropriate option. Some patients respond better to one mechanism than another, though predicting individual response is difficult. Both medications may cause nausea, diarrhoea, constipation, and other side effects, particularly during dose escalation. Injectable technique, injection frequency (both are once weekly), and storage requirements are similar for both.
Access in the UK is through NHS specialist tier 3 weight management services (with significant regional variation and waiting lists) or private prescribing following an online clinical assessment. Your GP can discuss which pathway is most appropriate for you. Use our free BMI calculator above to check your starting point, then discuss options 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 more effective than Wegovy?
Clinical trials showed Mounjaro achieved slightly higher average weight loss (up to 20.9% vs 14.9% with Wegovy), but individual results vary significantly. Some patients respond better to one medication than the other. A prescriber will assess your suitability based on your personal health profile, not just trial averages. Both are effective for weight management when combined with lifestyle changes.
Can I switch between Mounjaro and Wegovy?
Yes, switching is possible, but it must be managed by your prescriber. There is no official conversion formula — doses cannot be directly exchanged. Any switch involves stopping one medication and starting the other with appropriate timing and dose escalation. Never attempt to switch without medical guidance, particularly if taking other medications.
What's the main difference in how they work?
Mounjaro activates both GIP and GLP-1 hormone receptors (dual mechanism), while Wegovy activates only GLP-1 receptors (single mechanism). The dual action in Mounjaro may offer additional appetite regulation for some patients, but real-world response is highly individual. Your prescriber will recommend based on your medical history and any contraindications.
How long does it take to see results with either medication?
Weight loss typically becomes noticeable after 4–8 weeks, with more significant loss after 12–16 weeks. Both medications require ongoing dose escalation (each dose held for 4 weeks minimum) before reaching maintenance. Full clinical trial results were measured at 68–72 weeks. Progress varies; track your journey with our free progress tracker.
Are both medications available on the NHS?
Yes, both Mounjaro and Wegovy are licensed for NHS use through specialist tier 3 weight management services, but availability varies significantly by region. Waiting lists can be months to years in many areas. Private prescribing through UK-licensed online providers is faster but involves a fee. Your GP can advise on NHS eligibility and referral options for your area.
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.