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: Comparing Weight Loss Injections
Wegovy (semaglutide) and Mounjaro (tirzepatide) are both prescription-only weight loss injections licensed in the UK, but they work in different ways and have different starting doses. This guide compares both medications across mechanism, trial data, eligibility, and side effects — so you understand how they differ. Neither is automatically "better" — suitability depends on your individual health and is decided by your prescriber following a clinical assessment. Both are used alongside a reduced-calorie diet and increased physical activity.
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Enter your height and weight below. Supports both imperial (stone & lbs) and metric (kg). Results include NHS category, healthy weight range, and ethnicity adjustments.
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 They Work and Key Differences
Wegovy (semaglutide) is a GLP-1 receptor agonist — it activates one natural hormone pathway in the body. This helps reduce appetite, support feeling fuller after smaller meals, and slow how quickly food leaves the stomach. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways simultaneously. This dual mechanism means Mounjaro targets appetite control and blood sugar regulation through two separate pathways rather than one. Both are once-weekly injections. Wegovy starts at 0.25 mg and increases gradually to a maximum of 2.4 mg. Mounjaro starts at 2.5 mg and increases to a maximum of 15 mg. The different dose ranges reflect their different mechanisms and potencies — direct dose comparison is not meaningful. Both weekly weight loss injection options require a prescription and clinical assessment before starting.
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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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Clinical Trial Results and Individual Variation
In the STEP-1 clinical trial, Wegovy helped participants lose an average of approximately 14.9% of body weight over 68 weeks on the maximum 2.4 mg dose, alongside diet and exercise. In the SURMOUNT-1 clinical trial, Mounjaro helped participants lose an average of approximately 20.9% of body weight over 72 weeks on the maximum 15 mg dose, also with diet and exercise. However, individual results vary significantly — some people lose more, some less, and response depends on genetics, adherence, diet quality, and exercise. These are trial averages, not assured outcomes. Both medications show greater average weight loss than older GLP-1 medications like liraglutide (Saxenda). Your GP or prescriber will discuss realistic expectations for your situation. Use our free GLP-1 weight loss calculator to estimate potential results at different doses based on trial data.
- 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 weight loss injection is more effective?
Mounjaro showed slightly higher average weight loss in trials (20.9% vs 14.9%), but both are clinically effective. Individual response varies dramatically — some people respond better to one than the other. Your prescriber will recommend based on your personal health, not trial averages. Neither is universally "better"; suitability is individual.
Can I switch from Wegovy to Mounjaro if it's not working?
Yes, switching between medications is possible and is managed by your prescriber. There is no official conversion chart — your prescriber will assess the switch individually. You cannot self-switch or obtain both simultaneously. Always discuss any change with your prescriber first.
Are the side effects different between Wegovy and Mounjaro?
Both share common side effects: nausea, diarrhoea, constipation, vomiting, and stomach pain — especially when starting or increasing dose. These typically ease over time. Mounjaro may cause hair loss more frequently. Serious side effects (pancreatitis, gallbladder problems, allergic reactions) are rare but possible with both. Report any severe symptoms to your GP immediately.
How do I know if I'm eligible for Wegovy or Mounjaro?
Both require a BMI of 30 or above, or 27–29.9 with a weight-related health condition (diabetes, high blood pressure, sleep apnoea, cardiovascular disease). Lower BMI thresholds apply for South Asian, Chinese, Black African, or Black Caribbean adults. Eligibility is assessed during a clinical consultation — you cannot self-diagnose. Calculate your BMI using our free BMI calculator to get a starting point, then discuss with your GP.
Are Wegovy and Mounjaro available on the NHS?
Both are NHS-approved for specialist weight management services (tier 3/4), but regional availability varies significantly and waiting lists can be very long. Private prescription is available through online clinical assessment with a UK-licensed prescriber, dispensed by a GPhC-registered pharmacy. Access routes and costs differ — discuss options with your GP.
Ready to take control of your weight loss journey?
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