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 UK clinics and private prescribing services. They work differently, come in different doses, and show different results in clinical trials. Understanding how they compare can help you discuss options with your GP or a licensed prescriber. This guide covers the key differences that UK clinicians consider when assessing suitability for individual patients.
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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.
Clinical Trial Data and Mechanism Differences
Mounjaro (tirzepatide) works as a dual GIP and GLP-1 receptor agonist, activating two separate hormone pathways in the body. In the SURMOUNT-1 clinical trial, participants taking Mounjaro at the highest dose (15 mg) lost an average of approximately 20.9% of their body weight over 72 weeks, alongside a reduced-calorie diet and increased physical activity.
Wegovy (semaglutide) works as a GLP-1 receptor agonist only, activating a single hormone pathway. In the STEP-1 clinical trial, participants taking Wegovy at 2.4 mg lost an average of approximately 14.9% of their body weight over 68 weeks, also with diet and activity changes.
- Mounjaro: dual mechanism, higher average weight loss in trials
- Wegovy: single mechanism, established safety profile over longer clinical history
- Both: individual results vary significantly; trials show averages, not assures
Your prescriber will assess which mechanism may suit your individual health profile, medical history, and any existing conditions.
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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, Injection Schedules, and Individual Suitability
Mounjaro is dosed from 2.5 mg up to a maximum of 15 mg, given once weekly via a pre-filled KwikPen. Wegovy starts at 0.25 mg and escalates to a maximum of 2.4 mg, also once weekly. Both require a minimum of 4 weeks on each dose before increasing, allowing your body to adapt.
Neither medication is "better" universally — suitability depends on your BMI, weight-related health conditions, medical history, current medications, and how your body responds to treatment. Prescribers assess factors including contraindications, drug interactions, and likelihood of tolerating side effects. Some patients tolerate one medication better than the other. Your GP or licensed prescriber will determine which is appropriate for you following a proper clinical assessment.
Use our free BMI calculator above to check your current BMI, and try our GLP-1 weight loss calculator to see projected outcomes at different doses 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.
What's the main difference between Mounjaro and Wegovy?
Mounjaro is a dual GIP and GLP-1 receptor agonist (two hormone pathways), while Wegovy is a GLP-1 receptor agonist only (one pathway). Clinical trials showed higher average weight loss with Mounjaro, but individual results vary. Both require prescription and clinical assessment.
Which medication causes fewer side effects?
Both cause similar common side effects including nausea, diarrhoea, and constipation. Tolerability is individual — some patients tolerate one better than the other. Your prescriber will discuss side effect profiles and any concerns specific to your health history during assessment.
Can I switch from Wegovy to Mounjaro in a UK clinic?
Yes, switching is possible under prescriber guidance, though there is no official established conversion chart between them. Approximate equivalences exist based on clinical practice, but any switch must be managed by your GP or specialist to ensure safe dose adjustment and monitoring.
How do UK clinics decide between Mounjaro and Wegovy?
Assessment includes your BMI, weight-related health conditions, medical history, contraindications, drug interactions, and tolerability concerns. Both are licensed for weight management in UK adults meeting eligibility criteria. Your prescriber will recommend the most suitable option for you.
Do I need to be referred to an NHS clinic for Mounjaro or Wegovy?
For NHS access, you typically need referral to a tier 3 specialist weight management service through your GP. Private prescribing services offer both medications following an online clinical assessment, without NHS referral. Availability and waiting times vary by region.
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.