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 Prescribed vs Wegovy: What's the Difference?
If your GP or prescriber has mentioned Mounjaro (tirzepatide) or Wegovy (semaglutide) as a possible weight management option, you might wonder how these two prescription injections compare. Both are licensed for weight management in UK adults and available through NHS specialist services or private prescribing. The key difference lies in how they work: Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy is a GLP-1 agonist only. Understanding these differences can help you discuss the right option for your individual situation with your healthcare provider. Our GLP-1 weight loss calculator can help you explore potential outcomes at different doses.
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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: SURMOUNT vs STEP
Mounjaro was tested in the SURMOUNT-1 clinical trial, which evaluated tirzepatide across multiple doses over 72 weeks alongside a reduced-calorie diet and increased physical activity. Results showed average weight loss of approximately 15% at 5 mg, 19.5% at 10 mg, and 20.9% at 15 mg (the maximum dose).
Wegovy was evaluated in the STEP-1 clinical trial, which assessed semaglutide 2.4 mg (the maximum dose) over 68 weeks under similar lifestyle conditions. The trial showed average weight loss of approximately 14.9% body weight at the maximum dose.
Important context: These are average results from controlled clinical trials. Individual outcomes vary significantly based on genetics, adherence, lifestyle factors, and individual metabolism. Neither medication assures specific weight loss amounts, and results in real-world practice may differ from trial conditions.
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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.
How They Work: Mechanism Differences
Mounjaro (tirzepatide) activates two hormone pathways simultaneously: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual action may support appetite regulation, increase feelings of fullness, and assist with fat utilisation.
Wegovy (semaglutide) activates the GLP-1 pathway only. It helps reduce appetite, supports feeling fuller after smaller meals, and slows gastric emptying. GLP-1 agonists are well-established, having been used for type 2 diabetes management for several years before being adapted for weight management.
The dual mechanism of Mounjaro is theoretically more potent, which may explain slightly higher average weight loss in trials. However, both are effective options, and individual suitability depends on your personal health profile, not on mechanism alone.
- 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 injection is prescribed more often in the UK?
Both are available through NHS specialist weight management services and private prescribing. Wegovy has been available longer, so there may be more existing NHS pathways and GP familiarity. Mounjaro is newer but increasingly prescribed. Your prescriber will assess which is more suitable for your individual circumstances.
What are the main differences in dosing between Mounjaro and Wegovy?
Mounjaro starts at 2.5 mg weekly and escalates to a maximum of 15 mg weekly. Wegovy starts at 0.25 mg weekly and escalates to a maximum of 2.4 mg weekly. Both require a minimum of 4 weeks on each dose before increasing. The dosing schedules are managed differently by prescribers.
Are the side effects the same for Mounjaro and Wegovy?
Side effect profiles are very similar — both commonly cause nausea, diarrhoea, vomiting, constipation, and stomach pain, especially during dose escalation. Most side effects tend to reduce over time as your body adjusts. Discuss individual side effect risk with your prescriber based on your health history.
Can I switch from Mounjaro to Wegovy or vice versa?
Switching is possible but must be managed by your prescriber. There is no official established conversion chart — approximate equivalences exist (e.g., Mounjaro 10 mg ≈ Wegovy 1.7 mg), but any switch requires clinical reassessment and careful dose adjustment to ensure safety and effectiveness.
Which injection will my GP prescribe me?
Your prescriber will assess your individual health profile, medical history, BMI, weight-related conditions, and any contraindications to determine which medication is most suitable. Both are prescription-only and require a clinical assessment. Try our GLP-1 weight loss calculator to explore projected outcomes, then discuss the results with your healthcare provider.
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.