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 Is The Difference Between Mounjaro And Wegovy?
Mounjaro (tirzepatide) and Wegovy (semaglutide) are both prescription-only weight loss injections available in the UK, but they work differently and have distinct eligibility pathways. This guide explains how they differ in mechanism, dosing, clinical effectiveness, and NHS access — helping you understand which may be appropriate to discuss with your GP or a licensed prescriber following a clinical assessment.
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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.
Key Differences Between Mounjaro And Wegovy
How they work: Mounjaro is a dual GIP and GLP-1 receptor agonist — it activates two natural hormone pathways simultaneously. Wegovy is a GLP-1 receptor agonist only — it activates a single pathway. This dual mechanism in Mounjaro may explain why some clinical trials suggest slightly higher average weight loss, though individual results vary significantly.
Dosing: Mounjaro starts at 2.5 mg and escalates to a maximum of 15 mg weekly. Wegovy starts at 0.25 mg and reaches 2.4 mg weekly. Mounjaro's KwikPen contains four pre-filled doses; Wegovy's auto-injection pen also holds four weeks of doses.
Weight loss outcomes: In the SURMOUNT trial, Mounjaro 15 mg showed average weight loss of approximately 20.9% of body weight. Wegovy (semaglutide) in the STEP-1 trial showed approximately clinically studied weight loss at 2.4 mg. However, results vary — use our free GLP-1 weight loss calculator to explore projected outcomes based on your own starting weight.
Side effects: Both cause similar common side effects: nausea, diarrhoea, vomiting, and constipation. These typically improve as your body adjusts. Mounjaro side effects uk data mirrors Wegovy — gastrointestinal effects are most common early on.
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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 Access And UK Prescribing Routes
NHS eligibility: Both medications are available through NHS specialist tier 3 weight management services if you meet criteria: BMI of 30 or above, or BMI 27–29.9 with weight-related conditions like type 2 diabetes, hypertension, or sleep apnoea. However, NHS availability is limited by regional funding and waiting lists — access varies significantly across England, Scotland, Wales, and Northern Ireland.
Private prescribing: Both are available through private online prescribers without NHS referral. A licensed prescriber assesses your health via questionnaire and, if appropriate, issues a prescription dispensed by a GPhC-registered pharmacy. Private access is typically faster but involves out-of-pocket costs — roughly £150–£320 per month depending on dose.
NICE guidance: NICE has approved both tirzepatide (Mounjaro) and semaglutide (Wegovy) for weight management in eligible adults. Check the NICE NG220 guideline for the most current recommendations. Your GP can advise whether NHS tier 3 referral is appropriate for you, or whether private prescribing is an option you wish to explore.
- 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?
Clinical trials suggest Mounjaro may achieve slightly higher average weight loss (20.9% vs 14.9%), but both are effective. Individual results vary significantly based on genetics, adherence, diet, and activity level. A licensed prescriber can discuss which suits your circumstances best.
Can I switch from Wegovy to Mounjaro?
Yes, switching is possible but must be managed by your prescriber. There's no official conversion chart — approximate guidance suggests Wegovy 2.4 mg is comparable to Mounjaro 10 mg, though this is approximate. Any switch requires clinical assessment.
Which is available on the NHS in the UK?
Both Mounjaro and Wegovy are NICE-approved and available through NHS tier 3 specialist weight management services — but access is limited and varies by region. Waiting lists can be lengthy. Private prescribing offers faster access without NHS referral.
How long does it take to see results with Mounjaro or Wegovy?
Weight loss typically begins within 2–4 weeks as doses escalate. The dose titration phase lasts 16–20 weeks as your body adjusts. Most significant weight loss occurs between months 3 and 6. Sustained results depend on continued use alongside diet and activity changes.
What happens if I stop taking Mounjaro or Wegovy?
Weight loss is not permanent — most people gradually regain weight after stopping. These are ongoing treatments, not one-off supports. Your prescriber will discuss duration and long-term management with you. Some people use them long-term for sustained weight management.
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.