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 — UK Medication Comparison Guide
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections licensed in the UK, but they work differently and have distinct dosing schedules. This guide compares the two medications side-by-side so you understand the key differences. Both are GLP-1 related medications used alongside dietary changes and increased physical activity. Your prescriber will assess which is more suitable for your individual health needs after a clinical assessment.
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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 — Mechanism Differences
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two separate hormone pathways in your body. This dual action may help regulate appetite more comprehensively and support feeling fuller after smaller meals.
Wegovy (semaglutide) is a GLP-1 receptor agonist only — it activates one hormone pathway. This single mechanism still helps reduce appetite and supports weight management, but through one primary pathway rather than two.
- Mounjaro: dual GIP + GLP-1 agonist
- Wegovy: GLP-1 agonist only
- Both slow gastric emptying (how quickly food leaves your stomach)
- Individual response varies significantly
The clinical trials that tested these medications (SURMOUNT for Mounjaro, STEP for Wegovy) showed different average weight loss results, though both medications demonstrated meaningful outcomes when used alongside lifestyle changes.
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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, Prescriptions, and Clinical Trial Results
Mounjaro: Once-weekly injection, starting at 2.5 mg, escalating every 4 weeks to a maximum of 15 mg. In the SURMOUNT-1 trial, participants on 15 mg lost an average of approximately 20.9% of their body weight over 72 weeks — though individual results vary.
Wegovy: Once-weekly injection, starting at 0.25 mg, escalating to a maximum of 2.4 mg. In the STEP-1 trial, participants on 2.4 mg lost an average of approximately 14.9% of their body weight over 68 weeks.
Both medications require a prescription following clinical assessment by a UK-licensed prescriber. Access via the NHS involves referral to specialist weight management services; private access is available through online prescribing services. Use our free GLP-1 weight loss calculator to explore potential weight loss projections at different doses. Your GP can also discuss suitability based on your individual health profile.
- 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 suggest Mounjaro achieved a slightly higher average weight loss percentage than Wegovy in their respective trials (20.9% vs 14.9%). However, individual results vary significantly, and what works best depends on your personal health profile, medical history, and how your body responds. Your prescriber will assess which is most appropriate for you.
What's the main difference between Mounjaro and Wegovy?
Mounjaro is a dual GIP and GLP-1 receptor agonist (two hormone pathways), whilst Wegovy is a GLP-1 receptor agonist only (one pathway). Both are once-weekly injections. The dual mechanism in Mounjaro may offer different appetite regulation, but both medications support weight management when combined with lifestyle changes.
Can I switch from Wegovy to Mounjaro or vice versa?
Switching between medications requires prescriber guidance and must be managed carefully. There is no official established conversion chart between doses — your prescriber will assess your individual response and adjust accordingly. Never switch medications without medical supervision. Approximate clinical equivalence exists (Mounjaro 5 mg ≈ Wegovy 0.5 mg), but individual assessment is essential.
Do Mounjaro and Wegovy have the same side effects?
Both medications share common side effects including nausea, diarrhoea, constipation, and stomach pain — these are typical of GLP-1 medications. Individual experiences vary significantly. Review the full side effect profile with your prescriber or in the patient information leaflet before starting treatment.
How do I get Mounjaro or Wegovy prescribed in the UK?
Both are prescription-only and available via NHS specialist tier 3 weight management services (by referral from your GP) or private online prescribing services following clinical assessment. Eligibility requires a BMI of 30 or above, or 27+ with weight-related health conditions. Your prescriber will assess suitability individually.
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