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 and How Does It Work in the UK?
Mounjaro (tirzepatide) is a prescription-only weight management injection licensed in the UK for adults with obesity or overweight with weight-related health conditions. It works differently to other weight loss medications by activating two natural hormone pathways—GIP and GLP-1—which may help regulate appetite, reduce food cravings, and support feeling fuller after meals. Clinical trials showed that Mounjaro helped participants lose an average of 15–21% of their body weight when combined with a reduced-calorie diet and increased physical activity, though individual results vary significantly. Understanding how Mounjaro works and whether it's suitable for you requires assessment from a licensed prescriber.
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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 UK Eligibility Criteria
Mounjaro is a dual GIP and GLP-1 receptor agonist, which means it activates two separate hormone pathways in your body. This dual mechanism may be why it supports weight loss more effectively than single-mechanism medications. It works by signalling to your brain that you're full, reducing food cravings, slowing how quickly food leaves your stomach, and helping your body use fat more efficiently.
UK eligibility for Mounjaro includes:
- Adults with a BMI of 30 kg/m² or above (obesity)
- Adults with a BMI of 27–29.9 kg/m² (overweight) with weight-related health conditions such as type 2 diabetes, high blood pressure, sleep apnoea, or cardiovascular disease
- Adults of South Asian, Chinese, Black African, or Black Caribbean heritage may be eligible at lower BMI thresholds due to higher health risks at these levels
Use our free BMI calculator above to check if you might be in an eligible range, then discuss with your GP or a licensed prescriber who can assess your individual circumstances.
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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
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Mounjaro Doses, Side Effects, and What to Expect
Mounjaro is injected once weekly, starting at 2.5 mg and potentially increasing to a maximum of 15 mg over several months. The most common side effects are nausea, diarrhoea, constipation, vomiting, and stomach pain—many of which tend to reduce over time as your body adjusts. Serious side effects are rare but require immediate medical attention, including severe stomach pain (possible pancreatitis), gallbladder problems, or serious allergic reactions.
Mounjaro requires proper storage in a refrigerator (2–8°C) before use and can be kept at room temperature for up to 30 days after first use. Injection sites should be rotated weekly between the stomach, thigh, or upper arm. Always inform your prescriber of all other medications you're taking, as Mounjaro may affect how some oral medications are absorbed.
- 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 available on the NHS in the UK?
Mounjaro is approved for use within NHS specialist tier 3 and 4 weight management services. Access is through referral from your GP and availability varies significantly by region. Waiting times can be lengthy in many areas. Alternatively, Mounjaro is available privately through GPhC-registered pharmacies following an online clinical assessment with a UK-licensed prescriber.
What's the difference between Mounjaro and Wegovy?
Both are prescription-only weight loss injections licensed in the UK. Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist, while Wegovy (semaglutide) activates GLP-1 only. Clinical trials showed Mounjaro produced slightly higher average weight loss (15–21% vs ~15%), though results vary between individuals. Your prescriber will assess which is more suitable based on your health profile.
Can I get Mounjaro if I have type 2 diabetes?
Yes. Having type 2 diabetes or prediabetes is a weight-related health condition that may make you eligible for Mounjaro even at a lower BMI. Mounjaro can help with both weight management and blood sugar control. However, if you're already taking diabetes medications like insulin or sulphonylureas, your prescriber may need to adjust those doses, so always discuss your full medical history.
How much weight will I lose on Mounjaro?
Clinical trial data showed average weight loss of around 15–21% of body weight at the maximum dose, but individual results vary significantly. Some people lose more, some less. Weight loss depends on your starting weight, adherence to the medication, diet, physical activity, and individual metabolism. Your prescriber can discuss realistic expectations based on your personal circumstances.
What should I do if I miss a Mounjaro injection?
If you miss a dose within 4 days, take it as soon as you remember and continue your normal weekly schedule. If more than 4 days have passed, skip that dose and take your next injection on your normal scheduled day. Never take a double dose. The minimum time between injections should be 72 hours (3 days). Contact your prescriber if you're unsure.
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.