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 — How Do They Compare?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections licensed in the UK, but they work differently. Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy is a GLP-1 receptor agonist only. This guide compares both medications factually — mechanism, clinical trial data, dosing, side effects, and suitability. Both require a clinical assessment before prescription and must be used alongside lifestyle changes. Individual results vary significantly between patients.
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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 They Work and Trial Results
Mounjaro (tirzepatide) activates two hormone pathways — GIP (glucose-dependent insulinotropic polypeptide) and GLP-1. This dual mechanism may help regulate appetite more effectively. In the SURMOUNT-1 clinical trial, participants on the 15mg dose lost an average of 20.9% of their body weight over 72 weeks alongside reduced-calorie diet and increased physical activity.
Wegovy (semaglutide) is a GLP-1 receptor agonist only, activating a single pathway. In the STEP-1 trial, participants on 2.4mg (maximum dose) lost approximately 14.9% body weight over 68 weeks with the same lifestyle support.
Trial averages show Mounjaro produced numerically higher weight loss, but individual results vary considerably. Neither medication assures specific weight loss outcomes. Try our free GLP-1 weight loss calculator above to explore potential projections based on trial data, though your personal results will depend on your individual circumstances.
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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, Suitability, and Your Next Steps
Dosing schedules differ. Mounjaro starts at 2.5mg weekly and escalates to a maximum of 15mg weekly over approximately 20 weeks. Wegovy starts at 0.25mg weekly and reaches 2.4mg weekly over 16 weeks. Both are once-weekly injections. Mounjaro pens contain a higher concentration, and doses are not directly interchangeable with Wegovy.
Both medications are prescribed to adults with a BMI of 30 or above, or 27–29.9 with weight-related health conditions like type 2 diabetes, high blood pressure, or sleep apnoea. Eligibility varies by region in the UK. Your GP or a licensed prescriber will assess which medication (if any) is suitable for you individually. Neither is appropriate during pregnancy or for under-18s. Both require ongoing lifestyle changes — diet and physical activity — to achieve sustainable results.
Access in the UK is through NHS specialist tier 3 weight management services (where availability is variable and waiting lists can be long) or private prescribing following an online clinical assessment. Check your BMI using our free BMI calculator on this page to see whether you may meet eligibility criteria — then discuss options with your GP or a licensed prescriber. Both medications have common side effects including nausea, diarrhoea, and constipation, typically improving as your body adjusts. Serious side effects are rare but require urgent medical attention.
- 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 weight loss injection is more effective, Mounjaro or Wegovy?
Clinical trials show Mounjaro participants lost slightly more weight on average (approximately 21% vs 15%), but individual results vary significantly. Neither medication assures specific outcomes. Effectiveness depends on your personal circumstances, adherence, and lifestyle changes. Your prescriber will help you choose the most suitable option.
Can I switch from Wegovy to Mounjaro?
Switching between medications is possible and must be managed by your prescriber. There is no official established conversion chart — approximate equivalences exist based on clinical practice, but any transition requires medical supervision. Do not attempt to switch doses yourself; consult your GP or prescriber.
How much do Mounjaro and Wegovy cost in the UK?
NHS access is through specialist services where available (costs to you are covered if eligible). Private prescriptions range from approximately £129–£320 per month depending on dose and pharmacy. Mounjaro and Wegovy may have different pricing. Contact providers directly for current costs.
Are there side effects differences between Mounjaro and Wegovy?
Both share similar common side effects — nausea, diarrhoea, constipation, and indigestion. Individual tolerance varies. Serious side effects are rare with both but require urgent medical attention. Your prescriber will discuss potential side effects relevant to your health history.
Which medication is licensed in the UK right now?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are licensed in the UK for weight management in adults. Both are prescription-only and require clinical assessment. NICE has approved tirzepatide for NHS use within specialist weight management services, though regional availability varies.
Ready to take control of your weight loss journey?
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