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 Medication?
Mounjaro is a prescription-only weight management medication containing tirzepatide, a dual GIP and GLP-1 receptor agonist approved in the UK for adults with obesity or overweight with weight-related health conditions. It's a once-weekly injectable medication that works by activating two natural hormone pathways to help regulate appetite and support weight loss alongside a reduced-calorie diet and increased physical activity. This guide explains how Mounjaro works, UK eligibility criteria, available doses, and what to expect.
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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 to Expect
Mounjaro contains tirzepatide, which works differently from other weight loss injections because it activates both GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 receptor pathways — this dual mechanism may contribute to more significant appetite reduction. The medication signals to your brain that you feel fuller after eating smaller portions, slows how quickly food leaves your stomach, and helps regulate how your body uses fat. Each dose is delivered via a Mounjaro KwikPen, a pre-filled injection device you use once weekly. In clinical trials, adults taking Mounjaro lost an average of 15–21% of their body weight over 72 weeks, though individual results vary significantly. Most people start at 2.5 mg weekly, with doses gradually increased every 4 weeks up to a maximum of 15 mg, depending on tolerability and response. Use our free GLP-1 weight loss calculator above to explore potential weight loss projections at different doses.
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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.
UK Eligibility, Doses, and Prescription Access
In the UK, you may be eligible for Mounjaro 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, prediabetes, high blood pressure, high cholesterol, obstructive sleep apnoea, or cardiovascular disease. South Asian, Chinese, Black African, and Black Caribbean adults may qualify at lower BMI thresholds (27.5+) due to higher health risks at lower weights. Check your BMI using our free BMI calculator on this page. Mounjaro is prescription-only and available through NHS specialist weight management services or private online prescribing following clinical assessment. You cannot take Mounjaro if you're under 18, pregnant, breastfeeding, or have a history of medullary thyroid cancer. Your prescriber will assess suitability based on your individual health profile.
- 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 the same as Wegovy?
No. Both are prescription-only weight management injections, but Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, while Wegovy (semaglutide) is a GLP-1 agonist only. Clinical trials show Mounjaro resulted in approximately 15–21% average weight loss, compared to around 15% with Wegovy. Individual results vary significantly with both medications.
How do I get Mounjaro on the NHS?
Mounjaro is available through NHS specialist tier 3 weight management services. Ask your GP for a referral. Access varies by region and waiting lists can be lengthy. Private prescription is also available through online assessment with a UK-licensed prescriber dispensed by a regulated pharmacy.
What are the main side effects of Mounjaro?
Very common side effects include nausea, diarrhoea, vomiting, constipation, and stomach pain — these often improve over time. Less common are headache, fatigue, and injection site reactions. Serious side effects are rare but seek immediate medical attention for severe stomach pain (possible pancreatitis) or allergic reactions.
Can I switch from Wegovy to Mounjaro?
Yes, switching between weight loss medications is possible under prescriber guidance. There is no official conversion chart, but approximate equivalence exists (e.g., Mounjaro 10 mg ≈ Wegovy 1.7 mg). Any switch must be managed by your prescriber — do not change medications without clinical advice.
How long do I need to take Mounjaro?
Mounjaro is typically used long-term for sustainable weight management. Some people continue indefinitely, while others stop after reaching their goal. Your prescriber will advise based on your response and health needs. Stopping suddenly may result in weight regain, so any changes should be discussed with your healthcare provider.
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.