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?
Both Wegovy and Mounjaro are prescription-only weight loss injections licensed in the UK, but they work differently and have distinct dosing schedules. Wegovy (semaglutide) activates a single hormone pathway, whilst Mounjaro (tirzepatide) activates two. Understanding these differences helps you and your GP or prescriber decide which might be more suitable for your individual circumstances. Both are taken once weekly and require a clinical assessment before prescription. This guide compares them factually across mechanism, dosing, clinical evidence, and eligibility in the UK context.
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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.
How Wegovy and Mounjaro Work — Mechanism Differences
Wegovy (semaglutide) is a GLP-1 receptor agonist — it activates the GLP-1 hormone pathway, which signals fullness to the brain, slows stomach emptying, and supports appetite control. Clinical trials (STEP-1) showed Wegovy helped participants lose an average of approximately 14.9% of their body weight over 68 weeks at the maximum dose of 2.4 mg, alongside reduced-calorie diet and increased physical activity.
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways simultaneously. SURMOUNT-1 trial data showed average weight loss of approximately 20.9% of body weight at the maximum dose of 15 mg over 72 weeks with lifestyle support. The dual mechanism may offer slightly greater appetite suppression, though individual results vary significantly.
Both require a prescriber assessment and are not suitable for everyone. Use our free GLP-1 weight loss calculator to see potential weight loss projections at different doses of either medication.
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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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Dosing Schedules, Eligibility, and Individual Suitability
Wegovy doses: Start at 0.25 mg weekly, escalating every 4 weeks to a maximum of 2.4 mg. Mounjaro doses: Start at 2.5 mg weekly, escalating to a maximum of 15 mg. Both are once-weekly KwikPen injections. Eligibility is the same in the UK: BMI of 30 or above, or 27–29.9 with weight-related health conditions. Lower thresholds apply for South Asian, Chinese, Black African, or Black Caribbean adults. Which medication suits you depends on prescriber assessment of your health, medical history, and individual response. Neither is "better" — clinical data shows both support meaningful weight loss when combined with lifestyle change. Your GP or a licensed prescriber will assess whether Mounjaro, Wegovy, or another approach is most appropriate for 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.
Is Mounjaro more effective than Wegovy for weight loss?
Clinical trials show different average outcomes — Mounjaro SURMOUNT data averaged ~clinically studied weight loss, Wegovy STEP data averaged ~15% — but individual results vary significantly. Both are effective. Prescriber assessment of your health determines suitability, not trial averages alone.
Can I switch from Wegovy to Mounjaro or vice versa?
Switching between medications is possible but must be managed by your prescriber. There is no official conversion dose — the switch requires medical assessment and adjustment. Never change medications without prescriber guidance.
Are Wegovy and Mounjaro available on the NHS in the UK?
NICE has approved both for NHS specialist weight management services, but availability varies significantly by region and there are often long waiting lists. Private prescription access is available through UK-licensed prescribers and GPhC-registered pharmacies.
Which injection — Mounjaro KwikPen or Wegovy — causes fewer side effects?
Both cause similar common side effects: nausea, diarrhoea, constipation, and stomach discomfort, especially during dose escalation. Individual tolerance varies. Your prescriber can advise which is more suitable based on your health history.
Do I need to use your BMI calculator before seeing my GP about these medications?
Yes — our free BMI calculator helps you understand your starting point and eligibility thresholds. Check your BMI, then discuss weight management options with your GP or a licensed prescriber who can assess you properly.
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.