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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Wegovy or Mounjaro — UK Comparison Guide
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) is a GLP-1 receptor agonist, while Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist — this dual mechanism may explain why Mounjaro showed slightly higher average weight loss in clinical trials. Both require clinical assessment before prescription. The choice depends on your health profile, how you respond, and your access route — whether through NHS specialist services or private prescribing. Use our free GLP-1 weight loss calculator to explore potential outcomes at different doses.
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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 — Key Differences
Wegovy (semaglutide) activates the GLP-1 hormone pathway, which reduces appetite, promotes feeling fuller after smaller meals, and slows gastric emptying. It is a single-mechanism medication.
Mounjaro (tirzepatide) activates both GIP and GLP-1 pathways — this dual action may provide stronger appetite suppression and metabolic support. In the SURMOUNT-1 clinical trial, Mounjaro users lost an average of 15–21% of body weight, compared to approximately 15% with Wegovy in the STEP-1 trial, though individual results vary significantly.
- Wegovy: maximum dose 2.4 mg once weekly
- Mounjaro: maximum dose 15 mg once weekly using a Mounjaro KwikPen
- Both require dose escalation over 16–20 weeks
- Both are used alongside reduced-calorie diet and physical activity
Your GP or prescriber will assess which mechanism is more suitable for your individual health profile.
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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.
UK Access, Eligibility, and Side Effects
Both medications require a BMI of 30 or above, or 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or cardiovascular disease. NHS access is through specialist tier 3 weight management services — waiting lists vary by region. Private access is available through UK-licensed online prescribers following clinical assessment, with no NHS referral needed.
Both cause similar common side effects: nausea, diarrhoea, constipation, and stomach pain — usually during dose escalation. Serious side effects are rare but include severe abdominal pain (possible pancreatitis) and gallbladder issues. Individual tolerance differs; some users manage side effects better on one medication than the other. Track your progress with our free progress tracker and discuss any concerns with your prescriber immediately.
- 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 show Mounjaro achieved slightly higher average weight loss (15–21% vs ~15%), but individual results vary greatly. Effectiveness depends on your metabolism, lifestyle adherence, and how your body responds. Both are effective; the "best" choice is personal.
Can I switch from Wegovy to Mounjaro on the NHS?
Possibly, but this depends on your NHS specialist service and their protocols. Some services may offer a switch if you're not responding well to Wegovy. Discuss with your prescriber — they can assess whether switching is clinically appropriate for you.
How long does it take to see weight loss on Wegovy or Mounjaro?
Most users notice appetite reduction within 1–2 weeks, but significant weight loss typically takes 4–8 weeks. Clinical trials measured results over 68–72 weeks. Weight loss is gradual and sustainable — aim for 0.5–1 kg per week.
Do I need a prescription for both Wegovy and Mounjaro in the UK?
Yes, both are prescription-only medicines in the UK. You cannot buy them over the counter. You need a prescription from a licensed GP or prescriber following a clinical assessment of your health and eligibility.
What's the difference between Mounjaro and Ozempic?
Both contain semaglutide, but Ozempic is licensed for type 2 diabetes (up to 2 mg) and using it for weight loss is off-label. Wegovy is the licensed UK version for weight management (up to 2.4 mg). Mounjaro is a different medication (tirzepatide) and is also licensed for 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.