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 — Which Is Right for You?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections licensed in the UK. They work in similar ways but have important differences in mechanism, dosing, and trial results. This guide compares the two factually so you understand what your prescriber is assessing when deciding which medication may be suitable for you. Individual results vary significantly, and both require a clinical assessment before prescription.
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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 and Trial Data
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two natural hormone pathways that regulate appetite and blood sugar. In the SURMOUNT-1 clinical trial, participants on Mounjaro 15 mg lost an average of approximately 20.9% of their body weight over 72 weeks, whilst those on 10 mg lost around 19.5%.
Wegovy (semaglutide) is a GLP-1 receptor agonist only — it activates one hormone pathway. In the STEP-1 clinical trial, participants on Wegovy 2.4 mg lost an average of approximately 14.9% of body weight over 68 weeks.
- Mounjaro starting dose: 2.5 mg weekly, escalating to a maximum of 15 mg
- Wegovy starting dose: 0.25 mg weekly, escalating to a maximum of 2.4 mg
- Both are once-weekly injections — this is a key advantage over daily alternatives like Saxenda
- Clinical trial results were achieved alongside a reduced-calorie diet and increased physical activity — medication alone is not effective
The dual mechanism of Mounjaro may explain why trial data showed slightly higher average weight loss, but individual responses vary considerably and prescriber assessment determines suitability.
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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 Medication Is Right for You? — How Prescribers Decide
There is no universally "better" medication — suitability depends on your individual health profile, medical history, and how your body responds. Your prescriber will assess factors including your BMI, weight-related health conditions (such as type 2 diabetes or high blood pressure), any contraindications, and your preference for injection frequency and escalation schedule.
Both medications are prescription-only and require a clinical assessment. You cannot choose between them yourself; your prescriber will recommend the most appropriate option after reviewing your circumstances. Try our free GLP-1 weight loss calculator to estimate potential weight loss with different doses, and use our BMI calculator to confirm your current weight status. These tools provide context for conversations with your GP or prescriber about which medication may be right for you.
- 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 showed Mounjaro had slightly higher average weight loss (approximately 20.9% at maximum dose vs Wegovy's 14.9%), but individual results vary significantly. A medication that works well for one person may be less effective for another. Prescriber assessment determines which is more suitable for your individual circumstances.
What's the difference between GLP-1 and GIP+GLP-1?
Wegovy activates one hormone pathway (GLP-1 only). Mounjaro activates two pathways (GIP and GLP-1 together), which may explain its slightly higher trial results. Both are effective; the dual mechanism is one factor prescribers consider when choosing between them.
Can I switch from Mounjaro to Wegovy or vice versa?
Yes, switching is possible, but it must be managed by your prescriber. There is no official conversion chart between the medications. Any switch requires a new clinical assessment and prescription. Never stop one and start the other without prescriber guidance.
Does the NHS prescribe Mounjaro or Wegovy?
Both are available through NHS tier 3 specialist weight management services, but access varies significantly by region and availability. Waiting lists can be long. Private prescription access is faster but involves costs. Your GP can discuss NHS availability in your area.
Which has fewer side effects?
Both share similar side effect profiles (nausea, diarrhoea, vomiting, constipation). Individual tolerability varies — some people tolerate one better than the other. Side effects typically improve as your body adjusts. Your prescriber will discuss expected effects and management strategies with you personally.
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.