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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Wegovy vs Mounjaro: How Do They Compare?
Both Wegovy (semaglutide) and Mounjaro (tirzepatide) are prescription-only weight loss injections licensed in the UK, but they work differently and have distinct dose ranges. This guide compares their mechanisms, clinical trial data, eligibility criteria, and side effects to help you understand the key differences. Both require a prescription following clinical assessment by a qualified healthcare professional. Your GP or prescriber will decide which is more suitable for your individual circumstances.
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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.
Mechanism and Clinical Trial Outcomes
Wegovy (semaglutide) is a GLP-1 receptor agonist — it activates a single hormone pathway in the gut that signals fullness to the brain. In the STEP-1 clinical trial, participants taking Wegovy 2.4 mg (the maximum dose) lost an average of approximately 14.9% of their body weight over 68 weeks, alongside a reduced-calorie diet and increased physical activity.
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways simultaneously. In the SURMOUNT-1 clinical trial, participants on Mounjaro 15 mg (the maximum dose) lost an average of approximately 20.9% of body weight over 72 weeks under the same lifestyle conditions.
These are clinical trial averages. Individual results vary significantly based on genetics, lifestyle adherence, starting weight, and other health factors. Your prescriber will discuss realistic expectations for your situation.
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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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Doses, Side Effects, and Practical Differences
Dosing: Wegovy starts at 0.25 mg weekly and escalates to 2.4 mg maximum over approximately 16 weeks. Mounjaro starts at 2.5 mg and escalates to 15 mg maximum over approximately 20 weeks — doses are generally higher but still given once weekly.
Both can cause nausea, diarrhoea, constipation, and stomach pain, especially in the first weeks. These typically ease over time. Side effect profiles are broadly similar, though individual experiences vary.
Use our free GLP-1 weight loss calculator to compare projected weight loss outcomes at different doses. Both require ongoing prescriber monitoring and are typically used alongside dietary changes and increased physical activity — neither medication works without lifestyle effort.
How your prescriber decides: If you're eligible for either medication, your GP or specialist will choose based on your personal health profile, previous medication responses, any other medical conditions, and your preferences. Both are available through NHS specialist weight management services or private online prescribing in the UK.
Switching between medications is possible but must be managed by your prescriber — there's no standard conversion chart. If one isn't working well after 12 weeks at the target dose, your prescriber may suggest trying the other, or adjusting your dose within the same medication first.
- 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 causes more weight loss — Wegovy or Mounjaro?
Clinical trials suggest Mounjaro may result in slightly greater average weight loss (approximately 21% vs 15% at maximum doses), but individual results vary significantly. Some people lose more on Wegovy, others on Mounjaro. Your prescriber will recommend whichever is more suitable for your health profile.
Can I switch from Wegovy to Mounjaro or vice versa?
Yes, but only under your prescriber's supervision. A switch typically involves stopping one medication and allowing 3–7 days before starting the other, depending on the specific transition. Your prescriber will manage this safely and monitor you during the changeover.
Are Wegovy and Mounjaro available on the NHS in the UK?
Both are licensed for NHS use in England, Scotland, Wales, and Northern Ireland through specialist tier 3 weight management services. However, availability varies significantly by region and waiting lists can be lengthy. Private prescribing is available without NHS referral.
What's the difference between a GLP-1 agonist and a GIP/GLP-1 agonist?
Wegovy (GLP-1 only) activates one hormone pathway; Mounjaro (GIP/GLP-1) activates two pathways simultaneously. The dual action in Mounjaro may contribute to greater weight loss in clinical trials, though both work by reducing appetite and supporting fullness.
Do I need to stay on Wegovy or Mounjaro forever?
There's no set duration. Some people lose weight, reach their goal, and stop with support from their prescriber. Others continue long-term because weight returns if they stop. Your prescriber will discuss the best plan for maintaining your results.
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.