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 — Which Weight Loss Injection Is Better for UK Adults?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections licensed in the UK, but they work differently and have different NHS availability. Neither is objectively "better" — the right choice depends on your individual health, eligibility, and preferences. This guide compares both medications across mechanism, NHS access, effectiveness, and side effects to help you understand what each offers.
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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 They Work Differently and NHS Eligibility
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two hormone pathways simultaneously. This dual mechanism may give it an edge in appetite suppression and weight loss — clinical trials showed average weight loss of around 20.9% at the highest dose (15 mg per week). Mounjaro is now approved by NICE for use within NHS specialist weight management services, though availability varies significantly by region.
Wegovy (semaglutide) is a GLP-1 receptor agonist only, activating a single hormone pathway. Clinical trials (STEP-1) showed average weight loss of approximately 14.9% at the maximum dose (2.4 mg per week). Wegovy is also available through NHS tier 3 specialist services in some areas, but access remains patchy across the UK.
UK eligibility for both: 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 sleep apnoea. South Asian, Chinese, Black African, and Black Caribbean adults may qualify at lower BMI thresholds. Both require assessment by a licensed prescriber.
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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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NHS vs Private Access and Practical Differences
NHS access: Both Mounjaro and Wegovy are theoretically available through NHS tier 3 weight management services following GP referral. However, specialist services are under-resourced and waiting lists can be very long — in some areas, waits exceed 18 months. Availability depends on your local ICB (Integrated Care Board).
Private access: Both medications are available through private prescribers and GPhC-registered pharmacies without NHS referral, typically costing £129–£320 per month depending on dose. Mounjaro pens come as pre-filled KwikPen devices; Wegovy comes as pre-filled auto-injection pens.
Practical differences: Mounjaro has more dose steps (starting 2.5 mg, going up to 15 mg), allowing finer titration. Wegovy has fewer dose steps (0.25 mg to 2.4 mg). Both require once-weekly injections. Use our free GLP-1 weight loss calculator to estimate potential outcomes at different doses.
- 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 causes more side effects — Mounjaro or Wegovy?
Both have similar common side effects: nausea, diarrhoea, constipation, and stomach pain. These tend to improve after a few weeks as your body adjusts. Individual responses vary. Start with the lowest dose and increase gradually to minimise side effects. Your prescriber can adjust your dose based on how you tolerate it.
Is Mounjaro more effective than Wegovy for weight loss?
Clinical trials suggest Mounjaro may result in slightly greater average weight loss (20.9% vs 14.9%), but individual results vary considerably. Some people respond better to one than the other. "Better" depends on your personal health goals, side effect tolerance, and eligibility. A prescriber can discuss which suits your circumstances.
Can I get Mounjaro or Wegovy on the NHS in the UK?
Both are now approved by NICE for NHS use through tier 3 specialist weight management services, but availability is limited and waiting lists are very long in most areas. You'll need a GP referral. Private access is faster, typically within 1–2 weeks, but involves paying privately. Check your local NHS ICB for current availability.
What's the cost difference between Mounjaro and Wegovy privately?
Costs are broadly similar — typically £129–£320 per month depending on dose and pharmacy. Mounjaro's higher doses may cost slightly more. Prices vary between providers, so compare quotes. Some private services offer subscription discounts. Always check what's included in the quoted price.
How do I decide between Mounjaro and Wegovy?
A licensed prescriber will assess your health, eligibility, and preferences to recommend the most suitable option. Use our free BMI calculator to check your current status, then discuss with your GP or a private prescriber which medication aligns with your health goals and circumstances.
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.