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 15mg vs Wegovy — What's the Difference?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections licensed in the UK, but they work in different ways and have different dose ranges. Understanding how they compare can help you and your prescriber decide which might be suitable for you. This guide compares the mechanisms, clinical trial data, dose ranges, and eligibility criteria so you can make an informed decision about prescription weight loss treatment.
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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 Mounjaro and Wegovy Work — The Key Difference
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways at once. This dual mechanism may help regulate appetite more effectively by working on multiple signals in the body. In the SURMOUNT-1 clinical trial, participants taking Mounjaro 15mg (the maximum dose available in a KwikPen) lost an average of approximately 20.9% of their body weight over 72 weeks alongside a reduced-calorie diet and increased physical activity.
- Dual hormone action (GIP + GLP-1)
- Once-weekly injection
- Dose range: 2.5mg to 15mg
- Maximum effect typically at 15mg
Wegovy (semaglutide) is a GLP-1 receptor agonist only — it activates a single hormone pathway. The STEP-1 clinical trial showed participants taking Wegovy 2.4mg (the maximum licensed dose) lost an average of approximately 14.9% of body weight over 68 weeks with the same lifestyle support. Both are prescription-only and require clinical assessment before being prescribed.
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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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Which Is Right for You? — Individual Suitability
Both Mounjaro and Wegovy have the same UK eligibility criteria: a BMI of 30 or above, or 27–29.9 with weight-related health conditions (type 2 diabetes, high blood pressure, heart disease, or sleep apnoea). Your prescriber will assess your individual health history, current medications, and personal circumstances to determine which medication — if either — is suitable for you. Neither medication is universally "better" — results vary significantly between individuals, and what works well for one person may be less effective for another.
- Both require a clinical assessment before prescription
- Both are used alongside diet and exercise changes
- Both are taken once weekly via injection
- Individual response varies — trial averages do not predict personal outcomes
Use our free GLP-1 weight loss calculator to estimate potential weight loss at different doses, and our BMI calculator to check your starting point.
- 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 15mg more effective than Wegovy?
Clinical trial data suggests Mounjaro may produce slightly greater average weight loss (approximately 20.9% vs 14.9%), but individual results vary considerably. Some people respond better to one medication than the other. Your prescriber will consider your personal health profile to recommend the most suitable option.
What's the difference between tirzepatide and semaglutide?
Tirzepatide (Mounjaro) activates two hormone receptors (GIP and GLP-1), whilst semaglutide (Wegovy) activates one (GLP-1 only). The dual mechanism may explain slightly higher average weight loss with Mounjaro, but both are effective prescription treatments requiring clinical assessment.
Can I switch from Wegovy to Mounjaro 15mg?
Some people do switch medications under their prescriber's guidance, but there's no official established conversion chart. Any switch must be managed carefully by your GP or specialist to ensure safe dosing and monitoring. Never switch medications without medical supervision.
How long does it take to reach Mounjaro 15mg?
Mounjaro uses a gradual dose escalation schedule. Starting at 2.5mg, you typically increase by 2.5mg every four weeks — reaching 15mg takes approximately 16–20 weeks. This gradual approach helps your body adjust and minimises side effects.
Are both available through the NHS in the UK?
Both Mounjaro and Wegovy are available through NHS specialist tier 3 weight management services, though availability and waiting times vary significantly by region. Both are also available privately through UK-licensed online prescribers following a clinical assessment. Your GP can refer you to NHS services or advise on private options.
Ready to take control of your weight loss journey?
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