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 — NICE Guidelines Weight Loss Comparison
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription weight loss injections approved under NICE guidelines for UK adults with obesity or overweight with health conditions. They work differently — Mounjaro uses a dual GIP and GLP-1 mechanism, while Wegovy activates GLP-1 only. Clinical trials show both can support meaningful weight loss alongside lifestyle changes, though individual results vary significantly. This guide compares both medications factually to help you understand your options when discussing treatment with your GP or prescriber.
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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.
Clinical Trial Data & How They Work Differently
In the SURMOUNT-1 clinical trial, Mounjaro (tirzepatide) at 15 mg helped participants lose an average of approximately 20.9% of body weight over 72 weeks. Wegovy (semaglutide) at 2.4 mg showed average weight loss of around 14.9% over 68 weeks in the STEP-1 trial. Both medications were used alongside a reduced-calorie diet and increased physical activity.
How they differ:
- Mounjaro is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways simultaneously
- Wegovy is a GLP-1 receptor agonist only — it activates one pathway
- The dual mechanism may explain Mounjaro's numerically higher average weight loss in trials, though results vary between individuals
- Both slow gastric emptying and reduce appetite — the key difference is in the biological pathways activated
It's important to remember that these are average results from controlled clinical trials. Individual outcomes depend on many factors including baseline weight, adherence, diet, and exercise.
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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.
NICE Guidelines, Eligibility & How to Access Each
NICE has approved both Mounjaro and Wegovy for use in NHS weight management services for adults with a BMI of 30 or above, or BMI 27–29.9 with weight-related health conditions such as type 2 diabetes, hypertension, or sleep apnoea. Lower thresholds apply to South Asian, Chinese, Black African, and Black Caribbean adults due to higher metabolic risk at lower BMI.
Access routes in the UK:
- NHS: referral to tier 3 specialist weight management services — availability and waiting times vary by region
- Private: online consultation with a UK-licensed prescriber, if clinically suitable following assessment
- Both require prescription and are not available over-the-counter
Your GP or prescriber will assess which medication is more appropriate for you based on your health, preferences, and individual suitability. Use our free BMI calculator above to check if you fall within the eligibility range.
- 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 better — Mounjaro or Wegovy?
Both are effective, but "better" depends on individual factors. Mounjaro showed higher average weight loss in trials, but Wegovy is also clinically effective. Your prescriber will assess which suits you based on your health history, preferences, and potential side effects. Neither is universally superior — results vary between individuals.
How often do I inject Mounjaro and Wegovy?
Both are once-weekly injections. Mounjaro doses range from 2.5 mg to 15 mg weekly. Wegovy doses range from 0.25 mg to 2.4 mg weekly. Doses are increased gradually over weeks. Your prescriber will adjust the dose based on your response and tolerance.
Can I switch from Mounjaro to Wegovy or vice versa?
Yes, switching is possible, but only under prescriber guidance. There's no official conversion chart — approximate equivalences exist (e.g., Mounjaro 15 mg ≈ Wegovy 2.4 mg), but any switch requires medical assessment. Your prescriber will manage the transition carefully.
Are Mounjaro and Wegovy available on the NHS?
Yes, both are NICE-approved for NHS specialist weight management services. However, NHS access is through tier 3 referral services and availability varies significantly by region. Many areas have long waiting lists. Private access is available through online prescribing if you prefer faster availability.
What are the main side effects of Mounjaro and Wegovy?
Both commonly cause nausea, diarrhoea, vomiting, constipation, and stomach pain — especially when starting or increasing doses. These often improve over time. Serious side effects are rare but include severe stomach pain (possible pancreatitis), gallbladder issues, and allergic reactions. Always report any concerning symptoms to your prescriber. Try our GLP-1 weight loss calculator to explore potential outcomes with either medication.
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.