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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What is Mounjaro? The UK Guide to Tirzepatide
Mounjaro is a prescription-only weight management injection available in the UK that contains the active ingredient tirzepatide. It's a dual GIP and GLP-1 receptor agonist, which means it works on two natural hormone pathways to help regulate appetite and support weight loss. This medication is administered as a once-weekly injection and is approved for UK adults with a BMI of 30 or above, or 25+ with weight-related health conditions. Understanding how Mounjaro works, who can access it, and what to expect is essential before considering this treatment option.
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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 What Makes It Different
Mounjaro's unique dual mechanism sets it apart from other weight loss injections like Wegovy (semaglutide), which works on GLP-1 alone. Tirzepatide activates both GIP and GLP-1 receptor pathways, which may help regulate appetite more effectively by:
- Reducing hunger signals sent to the brain
- Increasing feelings of fullness after eating smaller meals
- Slowing how quickly food leaves your stomach
- Improving how your body uses fat for energy
Clinical trials (SURMOUNT-1) showed that on average, participants lost approximately 15–21% of their body weight over 72 weeks alongside a reduced-calorie diet and increased physical activity. However, individual results vary significantly. Mounjaro is not insulin and does not work like diabetes medication—it specifically targets appetite and satiety hormones. Use our free GLP-1 weight loss calculator above to estimate potential weight loss at different doses based on your starting weight.
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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 Eligibility, Doses, and UK Access
In the UK, you may be eligible for Mounjaro if you have a BMI of 30 kg/m² or above, or BMI 27–29.9 with weight-related conditions such as type 2 diabetes, prediabetes, high blood pressure, or cardiovascular disease. Lower thresholds (BMI 27.5+) apply to South Asian, Chinese, Black African, and Black Caribbean adults. Mounjaro is available on the NHS through specialist tier 3/4 weight management services (access varies by region), or privately through GPhC-registered online pharmacies following clinical assessment. Doses range from 2.5 mg up to 15 mg, administered once weekly. Calculate your BMI using our free BMI calculator above to see where you currently stand.
- 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.
Is Mounjaro (tirzepatide) available on the NHS in the UK?
Yes, Mounjaro is approved for use within NHS specialist weight management services. However, access is through tier 3/4 specialist services, not routine GP prescription. Availability varies significantly by region and ICB. Waiting lists can be lengthy in many areas. Private access is available through online prescribers.
What are the main side effects of Mounjaro?
Very common side effects include nausea, diarrhoea, vomiting, constipation, and stomach pain—especially during dose increases. These often settle after a few weeks. Common side effects include indigestion, fatigue, dizziness, and injection site reactions. Seek urgent medical attention for severe stomach pain, gallbladder problems, or serious allergic reactions.
How much weight will I lose on Mounjaro?
Clinical trials showed average weight loss of 15–21% of body weight over 72 weeks alongside lifestyle changes. Individual results vary significantly based on starting weight, adherence, diet, and exercise. Weight loss is not assured. Never expect specific kilogram outcomes—your results depend on many personal factors.
What's the difference between Mounjaro and Wegovy?
Mounjaro (tirzepatide) is a dual GIP+GLP-1 agonist, while Wegovy (semaglutide) is a GLP-1-only agonist. Mounjaro has a higher starting dose (2.5 mg) and maximum dose (15 mg). Clinical trials suggest slightly higher average weight loss with Mounjaro, but both are effective. Choice depends on individual suitability and prescriber guidance.
Do I need a prescription for Mounjaro in the UK?
Yes, Mounjaro is prescription-only in the UK. It requires a prescription from a UK-licensed prescriber following a clinical assessment. Your GP can refer you to NHS specialist services, or you can access private prescribing through online pharmacies. Never obtain Mounjaro without a valid prescription.
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.