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 Explained: NHS Guidance and How It Works
Mounjaro (tirzepatide) is a prescription-only weight management injection licensed in the UK that works differently from other slimming medications. This guide explains how Mounjaro works, what the NHS and NICE have approved it for, who is eligible, and the differences between NHS and private access routes in the UK. Whether you're considering this medication or simply want to understand the science behind it, this page covers everything you need to know about tirzepatide in a UK health context.
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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 Mounjaro Works and NHS Eligibility Criteria
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — different from other weight loss injections because it activates two natural hormone pathways, not just one. This dual action may help reduce appetite, support feeling fuller after smaller meals, and assist with regulating fat utilisation. The KwikPen device delivers a once-weekly injection, with doses ranging from 2.5 mg up to 15 mg maximum.
NHS eligibility in the UK: You may be eligible for Mounjaro through NHS specialist tier 3 weight management services if you have:
- A BMI of 30 kg/m² or above (obesity), OR
- A BMI of 27–29.9 kg/m² with a weight-related health condition (type 2 diabetes, prediabetes, high blood pressure, abnormal blood fats, sleep apnoea, or cardiovascular disease history)
Lower thresholds apply for adults of South Asian, Chinese, Black African, or Black Caribbean backgrounds (BMI 27.5 or above). NICE has approved tirzepatide for NHS use within specialist weight management services, though regional availability varies significantly across ICBs.
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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 Routes in the UK
NHS route: Access is through specialist tier 3 weight management services, typically requiring a GP referral. NICE guidance supports NHS prescribing, but waiting lists can extend several months to over a year in many areas depending on your ICB region. Once approved, the NHS covers the full cost of the medication.
Private route: Available through GPhC-registered online prescribers following an online clinical assessment. No NHS referral needed, and treatment can start sooner. Costs vary by dose (approximately £150–£300 per month). Both routes require a clinical assessment to confirm suitability. Calculate your potential weight loss with our GLP-1 weight loss calculator to understand possible outcomes at different doses, keeping in mind individual results vary significantly.
Clinical trial data (SURMOUNT-1) showed average weight loss of approximately 15–21% of body weight over 72 weeks on maximum doses, alongside a reduced-calorie diet and increased physical activity. However, individual results vary considerably. Common side effects include nausea, diarrhoea, and constipation — typically mild and improving over time. Your GP or prescriber can discuss whether Mounjaro is appropriate for your individual health situation and help you weigh the benefits against potential risks.
- 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.
What's the difference between Mounjaro and Wegovy?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist, whereas Wegovy (semaglutide) is a GLP-1-only medication. Clinical trials showed Mounjaro resulted in slightly higher average weight loss (15–21%) compared to Wegovy (approximately 15%). Both are prescription-only, once-weekly injections. The choice between them depends on individual health factors and prescriber assessment.
Do I need a GP referral to access Mounjaro on the NHS?
Yes, NHS access typically requires a GP referral to your local tier 3 specialist weight management service. Your GP will assess whether referral is appropriate based on your BMI and health conditions. Private prescribing does not require NHS referral — you can arrange a consultation directly with a GPhC-registered online prescriber.
What is dose titration with Mounjaro?
Dose titration is the gradual increase in Mounjaro dose over several weeks. You start at 2.5 mg weekly, then increase to 5 mg, 7.5 mg, 10 mg, 12.5 mg, and finally 15 mg. Each dose is maintained for at least 4 weeks before increasing. This approach allows your body to adjust and helps minimise side effects.
How long does it take to see weight loss results with Mounjaro?
Most people begin to notice changes within 2–4 weeks, though significant weight loss typically takes 12+ weeks. Clinical trials measured results over 72 weeks (approximately 17 months). Consistent use alongside a reduced-calorie diet and increased physical activity is essential for best results. Use our BMI calculator to track your current BMI and set realistic goals.
What happens if I stop taking Mounjaro?
Weight loss effects are typically reversed within weeks if you stop the medication — most people regain weight relatively quickly. Mounjaro is designed as an ongoing treatment, not a short-term course. Your prescriber will discuss long-term management and whether continued treatment is appropriate for your health situation.
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.