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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What's the difference between Wegovy and Mounjaro?
Wegovy and Mounjaro are both prescription-only weight management injections licensed in the UK, but they work differently and have distinct eligibility pathways through the NHS. Understanding how tirzepatide (Mounjaro's active ingredient) differs from semaglutide (Wegovy's active ingredient) can help you understand which medication might be appropriate for you. This guide covers the key differences in mechanism, dosing, NHS access, and clinical trial results in a UK context. Use our free GLP-1 weight loss calculator to explore potential outcomes at different doses, and our BMI calculator to check your personal eligibility baseline.
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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.
How Wegovy and Mounjaro work differently
Wegovy (semaglutide) is a GLP-1 receptor agonist — it activates one hormone pathway that signals fullness and reduces appetite. In STEP clinical trials, Wegovy showed average weight loss of around 14.9% of body weight over 68 weeks at the maximum dose of 2.4 mg once weekly.
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways simultaneously. This dual mechanism may explain why SURMOUNT trial data showed slightly higher average weight loss: approximately 20.9% at the maximum 15 mg dose. However, individual results vary significantly, and both medications work best alongside reduced-calorie diet and physical activity.
- Wegovy: single mechanism, escalates from 0.25 mg to 2.4 mg over 16 weeks
- Mounjaro: dual mechanism, escalates from 2.5 mg to 15 mg over 20+ weeks
- Both: once-weekly injections, require prescription, individual results vary
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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.
NHS eligibility and prescribing pathway in the UK
Both Wegovy and Mounjaro are approved by NICE for use within NHS specialist (tier 3) weight management services. Eligibility is identical: adults with a BMI of 30 kg/m² or above, or BMI 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or sleep apnoea. For South Asian, Chinese, Black African, or Black Caribbean adults, lower BMI thresholds apply (starting at 27.5). Access is through referral to specialist services, which vary by region and often have significant waiting times. Private prescribing offers faster access — both medications are available through UK-licensed prescribers following online clinical assessment. Check your personal eligibility using our BMI calculator, and discuss your options with your GP or prescriber to understand which route is available to you.
- 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 — Wegovy or Mounjaro?
Clinical trials showed Mounjaro (tirzepatide) produced slightly higher average weight loss than Wegovy (semaglutide) — approximately 20.9% versus 14.9% of body weight. However, individual results vary considerably, and many factors affect outcomes. Only a prescriber can advise which is more suitable for your specific health situation.
Can I switch from Wegovy to Mounjaro on the NHS?
Switching between medications is a clinical decision made by your prescriber or specialist service. There is no official NHS conversion chart — any switch requires assessment and re-titration. Discuss options with your tier 3 weight management service or private prescriber.
Are Wegovy and Mounjaro the same as Ozempic?
Wegovy and Ozempic both contain semaglutide, but Ozempic is licensed for type 2 diabetes at lower doses (up to 2 mg). Wegovy is the licensed weight management version at 2.4 mg. Mounjaro contains tirzepatide, a different active ingredient. Using Ozempic for weight loss is off-label in the UK.
How long do I need to take Wegovy or Mounjaro?
Both are ongoing medications — stopping typically leads to weight regain. Treatment duration depends on individual response, health goals, and prescriber guidance. Long-term use requires ongoing clinical review and monitoring.
What happens if I miss a dose of Wegovy or Mounjaro?
If you miss a dose within 4 days, take it as soon as remembered and continue your normal schedule. If more than 4 days have passed, skip that dose and take your next injection on schedule. Never double-dose. Contact your prescriber or pharmacist for specific advice.
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.