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
Free tracker included with every download this week
How Mounjaro Works To Lose Weight
Mounjaro (tirzepatide) is a prescription-only weekly weight loss injection that may help support weight management in UK adults. It works by activating two natural hormone pathways — GIP and GLP-1 — which help regulate appetite and promote feelings of fullness. Clinical trials showed participants lost an average of 15–21% of their body weight, though individual results vary significantly. Mounjaro is available through NHS specialist services or private prescription following a clinical assessment.
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Calculate Your BMI Instantly
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 Does Mounjaro Work for Weight Loss?
Mounjaro is a dual GIP and GLP-1 receptor agonist, which sets it apart from other weight loss injections. It activates two natural hormone pathways in your body:
- GIP (glucose-dependent insulinotropic polypeptide) — helps regulate appetite and food intake
- GLP-1 (glucagon-like peptide-1) — signals satiety to the brain, slowing stomach emptying and reducing hunger
Together, these mechanisms may help you feel fuller after smaller meals, reduce cravings, and support a reduced-calorie diet. In the SURMOUNT-1 clinical trial, participants taking 15 mg Mounjaro lost an average of 20.9% of their body weight over 72 weeks, alongside dietary changes and increased physical activity. Results are not assured — weight loss depends on individual metabolism, adherence to diet and exercise, and dose tolerance. You can estimate your potential weight loss using our free GLP-1 weight loss calculator, which projects results at different doses.
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No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
UK Eligibility and How to Access Mounjaro
In the UK, Mounjaro may be prescribed if you have a BMI of 30 or above, or a BMI of 27–29.9 with weight-related health conditions such as type 2 diabetes, hypertension, or cardiovascular disease. Check your BMI instantly using our free BMI calculator above. South Asian, Chinese, Black African, and Black Caribbean adults may be eligible at a lower BMI (from 27.5) due to higher health risks at these weights.
Access routes: NHS: referral to a tier 3 specialist weight management service (waiting lists vary by region). Private: online prescription from a UK-licensed prescriber following a clinical assessment. Mounjaro is supplied as a KwikPen (4 doses per pen, one injection per week). Track your progress with our free progress tracker as 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.
What's the difference between Mounjaro and Wegovy for weight loss?
Mounjaro (tirzepatide) is a dual GIP + GLP-1 agonist; Wegovy (semaglutide) is a GLP-1-only agonist. Clinical trials show Mounjaro may produce slightly higher average weight loss (up to 20.9% vs 14.9%), but both are effective. Individual results vary. Your prescriber will recommend which is more suitable for you.
How long does Mounjaro take to work for weight loss?
Most people notice appetite suppression within 1–2 weeks of the first injection. Measurable weight loss typically appears within 4 weeks. The full effect builds gradually as your dose increases (over 16–20 weeks to reach the 15 mg maximum). Sustained weight loss requires ongoing use alongside diet and exercise.
What's the starting dose of Mounjaro for weight loss in the UK?
The starting dose is 2.5 mg once weekly for the first 4 weeks. Your dose increases every 4 weeks — 5 mg, 7.5 mg, 10 mg, 12.5 mg, then 15 mg — based on tolerance and response. Your prescriber will guide dose titration. Not everyone reaches the maximum 15 mg dose.
Can I get Mounjaro on the NHS for weight loss in the UK?
Yes, NICE has approved tirzepatide (Mounjaro) for NHS weight management. Access is through tier 3 specialist weight management services — not your GP directly. Eligibility and availability vary significantly by region and NHS waiting lists can be lengthy. Private prescription is an alternative option.
What weight loss should I expect from Mounjaro?
Clinical trials showed average weight loss of 15–21% of body weight. This means a person weighing 100 kg might lose 15–21 kg over 72 weeks. However, individual results vary widely based on genetics, diet adherence, exercise, and metabolism. Some people lose more; others less. Results are not assured.
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.