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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Should I Start Mounjaro? A UK Comparison Guide
Deciding whether to start Mounjaro (tirzepatide) depends on your individual health profile, eligibility, and how you respond to treatment options. Both Mounjaro and Wegovy are prescription-only GLP-1 medications licensed for weight management in the UK, but they work differently and may suit different people. This guide compares both options to help you understand the key differences and what to discuss with your prescriber or GP.
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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.
Mounjaro vs Wegovy: Mechanism, Doses, and Trial Data
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two natural hormone pathways simultaneously, which may explain its slightly higher average weight loss in clinical trials. In the SURMOUNT-1 trial, participants on 15 mg Mounjaro lost approximately 20.9% of their body weight over 72 weeks alongside reduced-calorie diet and physical activity. Starting dose is 2.5 mg weekly, titrated up to a maximum of 15 mg.
Wegovy (semaglutide) is a GLP-1-only agonist with a single mechanism of action. The STEP-1 clinical trial showed approximately clinically studied weight loss at 68 weeks on the maximum 2.4 mg dose. Wegovy starts at 0.25 mg and escalates more gradually over 16+ weeks to reach maintenance.
Key differences:
- Mounjaro: dual mechanism, higher average weight loss (15–21%), faster titration (4 weeks per dose)
- Wegovy: single mechanism, proven safety track record, more gradual dose escalation
Important: these are clinical trial averages — individual results vary significantly, and neither medication assures weight loss.
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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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Is Mounjaro Right for You? Eligibility and Suitability
Both medications require the same UK eligibility criteria: BMI of 30 or above, or BMI 27–29.9 with at least one weight-related health condition (type 2 diabetes, high blood pressure, cardiovascular disease, or sleep apnoea). South Asian, Chinese, Black African, and Black Caribbean adults may qualify at lower BMI thresholds.
Your choice between Mounjaro and Wegovy should be made with your prescriber following a clinical assessment. They will consider your medical history, any medications you take, and whether you've tried other weight management approaches. Neither medication is suitable if you're pregnant, planning pregnancy, breastfeeding, or have certain medical conditions. Use our free GLP-1 weight loss calculator to explore potential outcomes at different doses, and our BMI calculator to understand your starting point — but remember, a prescriber must assess your individual suitability.
Both Mounjaro and Wegovy are prescription-only medicines available through NHS specialist tier 3 weight management services (with significant regional variation and waiting times) or through private online prescribing following clinical assessment. Side effects are similar between both: nausea, diarrhoea, constipation, and stomach pain are very common, especially at higher doses. Some people tolerate one better than the other — this is individual.
If you've used one medication previously, your prescriber may suggest switching to the other if you didn't achieve desired results or experienced intolerable side effects. There is no automatic equivalence between doses — any switch requires prescriber guidance. The key decision point is not which is "better" overall, but which is more suitable for your specific health profile and circumstances.
- 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 show Mounjaro (tirzepatide) achieved approximately 20.9% average weight loss at maximum dose, compared to Wegovy's 14.9%. However, individual results vary significantly, and some people respond better to one than the other. A prescriber will help determine which suits you based on your health profile, not trial averages.
Why does Mounjaro have a dual mechanism?
Mounjaro activates both GIP and GLP-1 hormone receptors, while Wegovy activates GLP-1 only. The dual mechanism may produce stronger appetite suppression and fat utilisation support in some people, but the clinical significance varies between individuals.
Is Mounjaro available on the NHS in the UK?
Mounjaro is approved by NICE for use within NHS specialist tier 3 weight management services. However, access varies significantly by region, and waiting lists can be very long. Private online prescribing is available as an alternative following clinical assessment.
Can I switch from Wegovy to Mounjaro?
Yes, but only following a clinical assessment by your prescriber. There is no official equivalence conversion between doses — switching requires your prescriber to assess your suitability and recommend an appropriate starting dose. Inform your prescriber of any side effects or outcomes with your previous medication.
How do I know which medication to start with?
You don't choose directly — your prescriber makes this assessment after evaluating your medical history, weight loss goals, previous treatments, and any contraindications. Both require a clinical consultation. Be honest with your prescriber about your health, medicines, and weight loss goals so they can recommend the most suitable option for you.
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.