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 available in the UK, but they work differently. Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy activates GLP-1 alone. Clinical trial data from SURMOUNT and STEP studies shows different average weight loss results, though individual outcomes vary significantly. Your prescriber will assess which medication suits your health profile and circumstances. Both require a clinical assessment before prescription and work best alongside a reduced-calorie diet and increased physical activity.
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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.
Clinical Trial Evidence and Mechanism Differences
The key difference lies in how these medications work. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two natural hormone pathways. Wegovy (semaglutide) activates only the GLP-1 pathway. In the SURMOUNT-1 trial, Mounjaro at 15 mg achieved average weight loss of approximately 20.9% of body weight over 72 weeks. In the STEP-1 trial, Wegovy at 2.4 mg achieved average weight loss of approximately 14.9% of body weight over 68 weeks. However, these are clinical trial averages — individual results vary considerably. Both medications are used with a reduced-calorie diet and exercise programme. Tirzepatide's dual mechanism may offer different benefits for some people, but clinical equivalence has not been established through direct head-to-head trials in the UK.
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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.
How Your Prescriber Chooses Between Them
Your prescriber will assess your medical history, current health conditions, medications, and personal circumstances to recommend the most suitable option. Both Mounjaro and Wegovy require you to have a BMI of 30 or above, or 27 or above with weight-related conditions such as type 2 diabetes, high blood pressure, or sleep apnoea. If you're taking oral medications or have specific health concerns, your prescriber may prefer one over the other. Both are available through NHS specialist weight management services (where funding allows) or through private prescribing with online assessment. Try our GLP-1 weight loss calculator to explore potential outcomes at different doses, and use our BMI calculator to understand your current health classification.
- 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 for weight loss?
Clinical trials show Mounjaro achieved higher average weight loss (20.9% at 15 mg) than Wegovy (14.9% at 2.4 mg). However, individual results vary significantly, and no direct head-to-head UK trial has compared them. Effectiveness depends on your personal response, adherence, diet, and exercise. Your prescriber will recommend the most suitable option based on your health profile.
What's the main difference between Mounjaro and Wegovy?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways. Wegovy (semaglutide) activates only GLP-1. This dual mechanism may produce different effects on appetite and blood sugar. Both require a prescription and clinical assessment in the UK.
Can I switch from Wegovy to Mounjaro or vice versa?
Yes, switching is possible, but it must be managed by your prescriber. There's no official established conversion chart between doses. Your prescriber will assess the switch carefully, considering your response to the current medication and any side effects. Never switch without medical guidance.
Do Mounjaro and Wegovy have the same side effects?
Both share common side effects including nausea, diarrhoea, vomiting, and constipation — particularly during dose escalation. The frequency and severity vary between individuals and doses. Serious side effects are rare but both require immediate medical attention if you experience severe stomach pain or allergic reactions.
Which medication is available on the NHS in the UK?
Both Mounjaro and Wegovy are approved for NHS use through specialist tier 3 weight management services, though availability varies significantly by region and ICB funding. Waiting lists can be lengthy. Both are also available privately through GPhC-registered pharmacies following an online 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.