The UK's most complete GLP-1 guide
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- 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 — Comparing UK Weight Loss Injections
If you're considering prescription weight loss medication in the UK, you may have heard about both Mounjaro and Wegovy. Both are licensed for weight management in UK adults, but they work slightly differently and have different clinical trial results. Understanding the key differences can help your prescriber assess which option might be most suitable for your individual health needs.
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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 Mounjaro and Wegovy Work Differently
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways in the body. This dual mechanism may help regulate appetite, reduce food cravings, and support feeling fuller after smaller meals. The GIP component is unique to Mounjaro—it works alongside the GLP-1 pathway to help your body utilise fat more effectively.
Wegovy (semaglutide) is a GLP-1 receptor agonist only, activating a single hormone pathway. It helps reduce appetite and signals fullness to the brain, but does not activate the GIP pathway. Both medications slow gastric emptying (how quickly food leaves the stomach), though this effect typically reduces over time.
In clinical trials, Mounjaro showed an average weight loss of approximately 15–21% of body weight, whilst Wegovy showed approximately 14.9% in the STEP-1 trial. Individual results vary significantly, and both are used alongside a reduced-calorie diet and increased physical activity. Use our free GLP-1 weight loss calculator to explore potential weight loss projections at different doses.
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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, Side Effects, and Prescriber Considerations
Dosing differences: Mounjaro starts at 2.5 mg and escalates to a maximum of 15 mg once weekly, with minimum 4-week intervals between dose increases. Wegovy starts at 0.25 mg and escalates to 2.4 mg maximum over 16 weeks. Both are once-weekly injections in pre-filled pens.
Side effects are similar for both medications: nausea, diarrhoea, constipation, and stomach pain are most common. Your GP or prescriber will assess which medication suits your individual health profile, considering your medical history, other medications, and personal health goals. Both require a clinical assessment before prescription, whether through NHS specialist weight management services or a private consultation with a UK-licensed prescriber.
- 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 or vice versa?
Any switch between medications must be managed by your prescriber. Whilst approximate dose equivalences exist (5 mg Mounjaro ≈ 0.5 mg Wegovy), there is no official conversion chart. Your prescriber will assess whether a switch is appropriate for your individual circumstances and manage the transition safely.
Which injection is better—Mounjaro or Wegovy?
Neither is universally "better"—suitability depends on your individual health profile. Mounjaro's dual mechanism appeals to some patients, whilst Wegovy's single-pathway approach suits others. Your prescriber will recommend based on your medical history, other medications, eligibility criteria, and personal health goals.
Are Mounjaro and Wegovy available on the NHS?
Both are licensed in the UK and available through NHS specialist tier 3 weight management services, though availability and waiting times vary significantly by region. Both are also available privately through UK-licensed prescribers following an online clinical assessment and prescription.
What's the difference between Ozempic and Wegovy?
Ozempic (semaglutide up to 2 mg) is approved for type 2 diabetes. Wegovy (semaglutide up to 2.4 mg) is the weight management version. Both contain semaglutide but in different doses and formulations. Ozempic is not licensed for weight loss in the UK—using it off-label for weight management is not recommended.
How do I know which medication is right for me?
Your prescriber will assess your BMI, weight-related health conditions, medical history, current medications, and personal preferences. Both require a clinical assessment. Use our free BMI calculator to understand your current BMI category, and discuss your eligibility and suitability with your GP or a licensed prescriber.
Ready to take control of your weight loss journey?
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