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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What's the Difference Between Wegovy and Mounjaro?
Both Wegovy and Mounjaro are prescription-only weight management injections available in the UK, but they work in different ways and have different dosing schedules. Wegovy (semaglutide) is a GLP-1 receptor agonist, whilst Mounjaro (tirzepatide) is a dual GLP-1 and GIP receptor agonist — meaning Mounjaro activates two hormone pathways instead of one. Both may help with weight management when used alongside a reduced-calorie diet and increased physical activity, but they're not interchangeable. Understanding the key differences can help you have an informed discussion with your GP or prescriber about which might be appropriate for you.
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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 Wegovy and Mounjaro Work Differently
Wegovy (semaglutide) is a GLP-1 receptor agonist — it activates a single hormone pathway called glucagon-like peptide-1. This may help reduce appetite, support feeling fuller after smaller meals, and slow how quickly food leaves your stomach. Clinical trials (STEP-1) showed that adults taking Wegovy 2.4 mg lost an average of approximately 14.9% of their body weight over 68 weeks, alongside lifestyle changes.
Mounjaro (tirzepatide) activates two pathways: both GIP (glucose-dependent insulinotropic polypeptide) and GLP-1. The dual action may provide additional appetite regulation and may support greater weight loss. Clinical trials (SURMOUNT-1) showed that participants on Mounjaro 15 mg lost an average of approximately 20.9% of their body weight at 72 weeks, also alongside reduced-calorie diet and increased physical activity.
Neither medication is assured to work — individual results vary significantly. Your prescriber will assess which is most suitable based on your personal health profile.
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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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Dosing Schedules and Eligibility
Wegovy starts at 0.25 mg once weekly and escalates over 16 weeks to a maximum of 2.4 mg once weekly. Mounjaro starts at 2.5 mg once weekly and escalates to a maximum of 15 mg once weekly, taking approximately 20 weeks to reach full dose.
Both are available in the UK for adults with a BMI of 30 or above, or 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or cardiovascular disease. South Asian, Chinese, Black African, and Black Caribbean adults may be eligible at lower BMI thresholds (27.5 and above) due to higher health risks at lower weights. Both require a clinical assessment and prescription — neither is available over the counter. You can access both through NHS specialist tier 3 weight management services or via private online prescribing following a licensed assessment.
- 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.
Can I switch from Wegovy to Mounjaro?
Switching between medications must be managed by your prescriber. There is no official conversion chart between Wegovy and Mounjaro doses, though approximate equivalences exist (e.g. Wegovy 2.4 mg is roughly equivalent to Mounjaro 15 mg). Any switch requires a clinical assessment to ensure it's safe and appropriate for you.
Which causes more side effects — Wegovy or Mounjaro?
Both have similar common side effects: nausea, diarrhoea, vomiting, constipation, and stomach pain. The frequency and severity vary between individuals and often decrease over time as your body adjusts. Your prescriber can advise on managing side effects and whether the medication is right for you.
Is Mounjaro more effective than Wegovy?
Clinical trials show Mounjaro achieved slightly higher average weight loss (approximately 20.9% vs 14.9%), but this is population-level data — individual results vary considerably. Effectiveness depends on many factors including adherence, lifestyle changes, and personal metabolism. Your prescriber will help determine which is most suitable for your situation.
Can I get either medication on the NHS?
Both are available through NHS specialist weight management services (tier 3), though access varies significantly by region and waiting lists can be long. You can also access both privately through a UK-licensed prescriber following an online clinical assessment. Costs, availability, and timelines differ depending on which route you choose.
What's the difference between Mounjaro and Ozempic?
Both contain semaglutide, but Ozempic (up to 2 mg) is licensed for type 2 diabetes, whilst Wegovy (up to 2.4 mg) is the licensed weight management version. Ozempic is not approved for weight loss in the UK — using it for weight loss is off-label prescribing. Always use the medication licensed for your specific condition.
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