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 vs Wegovy — What's the Difference?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections licensed in the UK, but they work in different ways. Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy is a GLP-1-only medication. Clinical trials showed different weight loss results for each, and individual suitability depends on your health profile and prescriber assessment. This guide compares both medications factually to help you understand the options available.
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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
Mounjaro (tirzepatide) activates two hormone pathways — GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual mechanism may help regulate appetite and fat utilisation more effectively. In the SURMOUNT-1 clinical trial, participants taking Mounjaro 15 mg lost an average of 20.9% of their body weight over 72 weeks, alongside a reduced-calorie diet and increased physical activity.
Wegovy (semaglutide) is a GLP-1-only receptor agonist. It activates one hormone pathway to reduce appetite and support feeling fuller after meals. In the STEP-1 clinical trial, participants taking Wegovy 2.4 mg lost an average of 14.9% of their body weight over 68 weeks, under similar lifestyle conditions.
Important: These are clinical trial averages — individual results vary significantly. Clinical trials do not directly compare the two medications head-to-head in identical populations, so direct superiority claims cannot be made. Your prescriber will assess which is more suitable for you.
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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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Doses, Eligibility, and What Matters for Your Decision
Mounjaro starts at 2.5 mg weekly and increases to a maximum of 15 mg, whilst Wegovy begins at 0.25 mg and reaches a maximum of 2.4 mg. Both are once-weekly injections. Eligibility is the same: you must have a BMI of 30 kg/m² or above, or BMI 27–29.9 with at least one weight-related health condition (such as type 2 diabetes, high blood pressure, or sleep apnoea). Use our free BMI calculator above to check where you stand. Both medications require a prescription and clinical assessment — your GP or prescriber will decide which is appropriate for your individual health profile, not on price or availability alone. Common side effects include nausea, diarrhoea, and constipation, though these often improve over time.
- 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 resulted in higher average weight loss (20.9% vs 14.9%) in their respective trials, but these were separate studies with different populations. No direct head-to-head comparison exists in the same group. Individual results vary greatly — some people respond better to one than the other. Your prescriber will assess which is more suitable for your health profile.
What's the main difference between Mounjaro and Wegovy?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, activating two hormone pathways. Wegovy (semaglutide) is GLP-1-only. The dual mechanism in Mounjaro may provide different appetite regulation, but both are effective weight loss medications. Your prescriber will advise which mechanism suits your individual needs.
Can I switch from Wegovy to Mounjaro on the NHS?
NHS access to weight loss medications is through specialist tier 3 weight management services, and switching between medications depends on your local ICB and specialist service. Private online prescribers may also offer switches. Discuss options with your GP or prescriber — any change must be managed by a healthcare professional.
Do Mounjaro and Wegovy have the same side effects?
Both medications share common side effects including nausea, diarrhoea, vomiting, constipation, and stomach pain. These are typically mild to moderate and often improve as your body adjusts. Serious side effects are rare but require immediate medical attention — such as severe stomach pain (possible pancreatitis) or gallbladder problems. Always report new symptoms to your prescriber.
Which medication is cheaper in the UK?
Private pricing varies by dose and provider, typically ranging from £129–£320 per month for either medication. NHS access is free but involves longer waiting times for specialist services. Availability and cost should not be the only factors — your prescriber will recommend the medication most suitable for your health profile.
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.