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 and Semaglutide — Understanding the Difference
Mounjaro and semaglutide are two different prescription weight management injections available in the UK, but they work in different ways. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, while semaglutide (sold as Wegovy for weight management) is a GLP-1 agonist only. Both are prescription-only and require a clinical assessment. Understanding how they differ can help you discuss options with your GP or a licensed prescriber. This guide explains UK eligibility, NHS access routes, and how these medications compare.
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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.
NHS Eligibility and Access Routes for Weight Management Treatment
NHS eligibility for prescription weight management treatment (including both Mounjaro and semaglutide injections) requires a BMI of 30 kg/m² or above, or 25–29.9 with weight-related health conditions such as type 2 diabetes, hypertension, or sleep apnoea. For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, lower thresholds apply — typically BMI 27.5 and above.
NHS access is through specialist tier 3 weight management services, not standard GP prescription. You'll need a referral from your GP to a local NHS weight management service. However, NHS availability varies significantly by region, and waiting lists can be long — sometimes months to years in some areas. NICE has approved both tirzepatide (Mounjaro) and semaglutide (Wegovy) for NHS use, but regional funding and service capacity determine actual availability.
Private access is available through online consultations with UK-licensed prescribers, followed by a prescription dispensed by a GPhC-registered pharmacy. This route bypasses NHS waiting lists but costs approximately £129–£320 per month depending on dose and provider.
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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.
Mounjaro and Semaglutide — How They Differ and Expected Results
Mounjaro (tirzepatide) works on two hormone pathways — GIP and GLP-1 — which may explain why clinical trial results showed average weight loss of approximately 15–21% of body weight over 72 weeks. Semaglutide (Wegovy) works on GLP-1 alone and showed average weight loss of approximately 14.9% over 68 weeks in trials. Both medications are injected once weekly and require dose titration over several weeks.
Mounjaro's dual mechanism may offer a slight advantage for some patients, but individual results vary significantly. Both require a reduced-calorie diet and increased physical activity to be effective. Common side effects include nausea, diarrhoea, and constipation — typically mild and temporary. Your prescriber will help you choose which medication suits your health profile best. Calculate your potential weight loss trajectory using our GLP-1 weight loss calculator, and track your progress with our free progress tracker once you begin treatment.
- 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 get Mounjaro or semaglutide on the NHS?
Yes — both Mounjaro (tirzepatide) and Wegovy (semaglutide) are NICE-approved for NHS use. However, access is through specialist tier 3 weight management services, not standard GP prescription. You need a referral from your GP, and availability varies by region. Waiting lists can be months to years. Private access is faster but involves a cost.
What's the difference between Mounjaro and semaglutide?
Mounjaro contains tirzepatide, a dual GIP+GLP-1 receptor agonist. Semaglutide (Wegovy) is a GLP-1 agonist only. Mounjaro showed slightly higher average weight loss in trials (15–21% vs 14.9%), but both are effective. Your prescriber will assess which suits your individual health profile best.
How do I check if I'm eligible for NHS weight management treatment?
You're eligible if your BMI is 30 or above, or 27–29.9 with weight-related health conditions (type 2 diabetes, high blood pressure, sleep apnoea, etc.). For South Asian, Chinese, Black African, or Caribbean adults, eligibility may apply at a lower BMI. Your GP can assess this and refer you to specialist services.
How long does it take to see results with these injections?
Weight loss typically begins within 4–8 weeks, but significant results develop over 12–24 weeks as doses increase. Clinical trials ran for 68–72 weeks. Results depend on adherence to diet and exercise, dose reached, and individual metabolism. Track your progress using our free progress tracker.
What if the NHS waiting list is too long?
Private prescribing is an alternative. Licensed UK prescribers offer online consultations, and GPhC-registered pharmacies dispense medication without NHS referral. This usually takes 1–2 weeks. It costs more but bypasses NHS waiting times. Ensure any provider is GPhC-registered and requires a proper clinical assessment.
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.