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 15ml vs Wegovy — What's the Difference?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections available in the UK. The 15ml KwikPen is Mounjaro's maximum dose. While they work similarly by suppressing appetite and helping you feel fuller, they use different mechanisms. This guide compares both medications side-by-side so you understand the key differences — and how to discuss eligibility with your GP or prescriber. Both require a clinical assessment, and individual suitability depends on your personal health profile, not on which drug is "better".
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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
Mechanism: Mounjaro works as a dual GIP and GLP-1 receptor agonist, activating two natural hormone pathways. Wegovy is a GLP-1 receptor agonist only, activating a single pathway. This dual action in Mounjaro may explain slightly higher average weight loss in clinical trials.
Clinical trial results (SURMOUNT-1 vs STEP-1): Mounjaro at 15mg showed average weight loss of approximately 20.9% body weight over 72 weeks. Wegovy at 2.4mg (maximum dose) showed average weight loss of approximately 14.9% body weight over 68 weeks. Both trials included reduced-calorie diet and increased physical activity alongside medication. Important: these are trial averages — individual results vary significantly, and weight loss is never assured.
Dosing also differs: Mounjaro escalates from 2.5mg to 15mg over 20+ weeks. Wegovy escalates from 0.25mg to 2.4mg over 16+ weeks. Both are once-weekly injections.
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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
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Which Is Right for You? Prescriber Assessment
Choosing between Mounjaro 15ml and Wegovy isn't about superiority — it's about individual suitability. A prescriber will assess factors including your BMI, weight-related health conditions (type 2 diabetes, high blood pressure, cardiovascular disease), other medications you take, and any contraindications. Both medications require prescription and a clinical assessment. UK prescribing access is through NHS specialist tier 3 weight management services or private online prescribing following an online clinical consultation.
Your prescriber may recommend one over the other based on your health profile, not on trial data alone. Use our free GLP-1 weight loss calculator to understand projected weight loss at different doses, and discuss both options with your GP or prescriber during your assessment.
- 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.
Why does Mounjaro 15ml show higher weight loss than Wegovy in trials?
Mounjaro's dual GIP and GLP-1 mechanism activates two appetite-regulating pathways, versus Wegovy's single GLP-1 pathway. The SURMOUNT-1 trial showed Mounjaro 15mg at approximately clinically studied weight loss versus Wegovy's 14.9% in STEP-1. However, trial results don't predict individual outcomes — results vary significantly between people.
Is Mounjaro 15ml or Wegovy available on the NHS in the UK?
Both are available through NHS specialist weight management services (tier 3), but access varies significantly by region and waiting lists are often long. Private prescription access is available through GPhC-registered online prescribers without NHS referral. Your GP can discuss NHS eligibility with you.
Can I switch from Wegovy to Mounjaro 15ml?
Yes, switching between medications is possible and would be managed by your prescriber. Approximate dose equivalence exists (Wegovy 2.4mg ≈ Mounjaro 10mg roughly), but there is no official conversion chart. Any switch requires prescriber guidance and a clinical assessment.
What's the difference between Mounjaro's mechanism and Wegovy's?
Mounjaro is a dual GIP and GLP-1 agonist (activates two hormone pathways). Wegovy is a GLP-1 agonist only (single pathway). The dual action may explain Mounjaro's slightly higher average weight loss in trials. Both suppress appetite and help you feel fuller after eating.
Which medication has fewer side effects, Mounjaro 15ml or Wegovy?
Side effect profiles are similar — both commonly cause nausea, diarrhoea, vomiting, and constipation, especially during dose escalation. Nausea often improves over time. Side effect severity varies individually. Your prescriber will discuss potential side effects and how to manage them during your assessment.
Ready to take control of your weight loss journey?
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