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 — What's the Difference?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections available in the UK, but they work differently and may suit different people. If you're exploring weight management options as a new customer, understanding how these two medications compare can help inform conversations with your GP or a licensed prescriber. This guide breaks down the key differences in how they work, their clinical trial results, and what eligibility looks like in the UK.
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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 — The Science Behind Each
The fundamental difference between these medications lies in their mechanisms of action. Wegovy (semaglutide) is a GLP-1 receptor agonist — it activates a single hormone pathway (GLP-1) that helps regulate appetite and blood sugar. Mounjaro (tirzepatide), however, is a dual GIP and GLP-1 receptor agonist — it activates two pathways simultaneously, which may provide additional appetite suppression and metabolic support.
Clinical trial data shows the difference in outcomes. In the SURMOUNT-1 trial, Mounjaro at the maximum 15 mg dose resulted in approximately 20.9% average body weight loss over 72 weeks. In comparison, Wegovy's STEP-1 trial showed approximately 14.9% average body weight loss at the maximum 2.4 mg dose over 68 weeks. However, individual results vary significantly, and what works best depends on your personal health profile, not trial averages.
Both medications slow gastric emptying (how quickly food leaves your stomach) and require ongoing use alongside dietary changes and physical activity. Your prescriber will assess which mechanism may suit your body's response better.
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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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Eligibility, Doses, and Practical Differences
In the UK, eligibility for both medications is similar: a BMI of 30 or above, or BMI of 27–29.9 with weight-related health conditions such as type 2 diabetes or high blood pressure. The key practical difference is injection frequency and dose progression. Mounjaro ranges from 2.5 mg to 15 mg once weekly, with a longer titration schedule. Wegovy ranges from 0.25 mg to 2.4 mg once weekly over approximately 16 weeks. Both are prescription-only and require a clinical assessment before being prescribed — availability varies between NHS specialist services and private prescribers. Your GP or licensed prescriber will recommend which is more appropriate based on your health history, any other medications, and how your body responds during the initial weeks of treatment.
- 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 medication causes fewer side effects — Mounjaro or Wegovy?
Both medications cause similar common side effects: nausea, diarrhoea, vomiting, and constipation. Individual responses vary significantly. Some people tolerate one better than the other. This is why prescribers may recommend starting one and switching if side effects are problematic — a decision made during clinical review after 4–8 weeks.
Can I switch from Wegovy to Mounjaro if I'm not getting results?
Yes, switching is possible, but only under prescriber guidance. There is no official established conversion chart — your prescriber will assess your current response and may recommend starting Mounjaro at a lower dose. Any switch requires a new clinical assessment and prescription.
How quickly will I see weight loss with Mounjaro or Wegovy?
Most people notice appetite changes within the first 2–3 weeks. Measurable weight loss typically becomes apparent after 4–6 weeks. Clinical trials tracked results over 68–72 weeks, so these medications work gradually alongside lifestyle changes. Use our free GLP-1 weight loss calculator to estimate potential outcomes at different doses.
Are new customers eligible for special offers on these medications?
Pricing and promotional offers vary by provider. NHS access is free but involves referral to specialist services and potential waiting lists. Private prescribers may offer introductory pricing or payment plans — always check directly with the prescriber about current offers and what's included.
How do I know which medication is right for me?
This is a clinical decision made with your GP or licensed prescriber after assessment. They will consider your BMI, health conditions, previous medication responses, and any other medications you take. Use our free BMI calculator to check your current BMI category and discuss the results with your prescriber at consultation.
Ready to take control of your weight loss journey?
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