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
Free tracker included with every download this week
Wegovy vs Mounjaro: What's the Difference?
Both Wegovy and Mounjaro are prescription-only weight loss injections licensed in the UK, but they work in different ways. Wegovy (semaglutide) is a GLP-1 receptor agonist, while Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — meaning it activates two hormone pathways instead of one. Clinical trials show both can support weight loss when used alongside a reduced-calorie diet and increased physical activity, though individual results vary significantly. Your GP or prescriber will assess which, if either, is suitable based on your personal health profile and medical history.
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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 Clinical Trial Results
Wegovy (semaglutide) activates only the GLP-1 receptor pathway. In the STEP-1 clinical trial, participants taking 2.4 mg once weekly lost an average of approximately 14.9% of their body weight over 68 weeks, alongside dietary and lifestyle changes.
Mounjaro (tirzepatide) activates both GIP and GLP-1 receptors — this dual mechanism is its key difference. In the SURMOUNT-1 trial, participants on the 15 mg dose (maximum) lost an average of approximately 20.9% of body weight over 72 weeks with the same lifestyle support.
Both medications may help reduce appetite, increase feelings of fullness, and slow how quickly food leaves the stomach. Trial results show Mounjaro achieved numerically higher average weight loss, but individual outcomes vary considerably. Neither medication assures a specific weight loss outcome.
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Doses, Side Effects, and Individual Suitability
Wegovy doses start at 0.25 mg and escalate to a maximum of 2.4 mg once weekly. Mounjaro doses range from 2.5 mg to 15 mg once weekly. Both use a gradual titration schedule (increasing slowly over weeks) to minimise side effects.
Common side effects of both include nausea, diarrhoea, vomiting, and constipation — most mild and temporary as your body adjusts. Which medication is prescribed depends on your BMI, weight-related health conditions, medical history, other medications, and individual prescriber assessment. There is no standard "conversion" between doses — if switching medications, your prescriber will assess and adjust independently. Use our free GLP-1 weight loss calculator to explore potential outcomes, and our BMI calculator to check your starting point.
- 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 achieved higher average weight loss (approximately 20.9% vs 14.9%), but results vary significantly between individuals. "More effective" depends on your personal response — some people respond better to Wegovy, others to Mounjaro. Your prescriber will help determine which is more suitable for you based on your health profile, not just trial averages.
Can I switch from Wegovy to Mounjaro or vice versa?
Yes, switching is possible and done by prescribers in clinical practice, but there is no official established conversion chart. Your prescriber will assess your response to the first medication and determine an appropriate starting dose for any switch. This requires a new clinical assessment — never attempt to switch doses yourself.
Which is available on the NHS?
Both Mounjaro and Wegovy are approved by NICE for NHS use, but access varies significantly by region and depends on tier 3 specialist weight management services. Waiting lists can be lengthy in many areas. Private access is available through GPhC-registered online pharmacies following clinical assessment, without NHS referral.
Are there differences in side effects between the two?
Both medications cause similar common side effects: nausea, diarrhoea, constipation, and stomach pain. Individual tolerance differs — some people tolerate one better than the other. Serious side effects are rare but can include pancreatitis symptoms (severe stomach pain). Always report side effects to your prescriber.
What if neither Wegovy nor Mounjaro works for me?
Other prescription weight loss options exist in the UK, including Saxenda (liraglutide, a daily GLP-1 injection). Your prescriber may also explore bariatric surgery via NHS tier 4 services if appropriate. Weight loss support is individualised — your healthcare team will work with you to find the right approach.
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.