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
What is Mounjaro Injection?
Mounjaro is a prescription-only, once-weekly weight loss injection licensed in the UK for adults with obesity or overweight with related health conditions. It contains tirzepatide, a dual GIP and GLP-1 receptor agonist — meaning it works on two natural hormone pathways to help reduce appetite and support weight management alongside diet and exercise. Unlike some other weight loss medications, Mounjaro's dual mechanism may help you feel fuller after smaller meals and support better appetite control throughout the day.
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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 Does Mounjaro Work?
Mounjaro activates two hormone pathways simultaneously: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual action sets it apart from single-mechanism weight loss injections. When you inject Mounjaro weekly, it signals to your brain that you're satisfied sooner, reduces food cravings, and slows how quickly food leaves your stomach — helping you eat less without feeling deprived.
The medication doesn't contain insulin and doesn't work like insulin. Instead, it mimics natural hormones your gut produces after eating. Clinical trials showed that on average, adults using Mounjaro alongside a reduced-calorie diet and increased physical activity lost between 15% and 21% of their body weight — though individual results vary significantly. Use our free GLP-1 weight loss calculator 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.
Mounjaro Doses and Eligibility in the UK
Mounjaro comes as a weekly injection in a pre-filled KwikPen device. Doses start at 2.5 mg and gradually increase to a maximum of 15 mg weekly, with a minimum of 4 weeks between dose increases. In the UK, you may be eligible 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, high blood pressure, sleep apnoea, or heart disease. People of South Asian, Chinese, Black African, or Black Caribbean heritage may qualify at lower BMI thresholds due to higher health risks at these weights.
Access in the UK is through NHS specialist weight management services (often with waiting lists) or private online prescribing following a clinical assessment. A prescriber will review your medical history and health conditions before issuing a prescription. Check your personal BMI and eligibility using our free BMI calculator above.
- 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 are the main side effects of Mounjaro injection?
Very common side effects include nausea, diarrhoea, vomiting, constipation, and stomach pain — especially when starting or increasing doses. Most side effects are temporary and improve over time. Less common side effects include fatigue, dizziness, headache, and injection site reactions. Seek urgent medical attention for severe stomach pain (possible pancreatitis), serious allergic reactions, or gallbladder problems. Always report side effects to your prescriber.
How much weight can I expect to lose on Mounjaro?
Clinical trials (SURMOUNT-1) showed average weight loss of 15% to 21% of body weight over 72 weeks on Mounjaro, alongside a reduced-calorie diet and exercise. Individual results vary considerably — some people lose more, others less. Weight loss depends on your starting weight, adherence to diet and activity changes, and how your body responds. Results are not assured, and stopping the medication typically leads to weight regain without sustained lifestyle changes.
Is Mounjaro available on the NHS in the UK?
Yes, Mounjaro (tirzepatide) is approved for use within NHS specialist weight management services (tier 3/4). However, access varies significantly by region and waiting lists can be lengthy in many areas. You'll need a referral from your GP to a specialist service. Private access is available through online prescribing services following a clinical assessment, though this incurs private costs ranging from approximately £200–£350 per month depending on dose.
Can I switch from Wegovy to Mounjaro?
Yes, some people switch between these medications, but it must be managed by your prescriber. Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist, while Wegovy (semaglutide) is a GLP-1-only injection. Approximate dose equivalences exist (Mounjaro 5 mg ≈ Wegovy 0.5 mg), but there's no official conversion chart. Your prescriber will determine the safest approach based on your response to previous treatment and current health.
How do I store Mounjaro pens?
Unopened Mounjaro pens must be stored in a refrigerator at 2–8°C and kept in their original carton away from light — don't freeze them. Once you start using a pen, you can keep it at room temperature (below 30°C) for up to 30 days. Dispose of the pen 30 days after first use, even if medication remains. Always check the expiry date before using and never use frozen pens.
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.