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 a Mounjaro Jab?
A Mounjaro jab is a once-weekly injectable prescription medication containing tirzepatide, licensed in the UK for weight management in adults. The injection works as a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways to help reduce appetite and support feeling fuller after meals. It's administered using a pre-filled KwikPen device and is available through NHS specialist weight management services or private prescribing following a clinical assessment. Individual results vary, but clinical trials showed participants lost an average of 15–21% of their body weight when used alongside dietary changes and increased physical activity.
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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 the Mounjaro Injection Works
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — a unique mechanism that sets it apart from other weight loss injections like Wegovy (semaglutide), which works as a single GLP-1 receptor agonist. When you take a Mounjaro jab each week, the medication activates two natural hormone pathways in your body:
- GLP-1 signalling helps regulate appetite and promotes feelings of fullness
- GIP signalling helps control glucose metabolism and may enhance fat utilisation
The injection slows gastric emptying — how quickly food leaves your stomach — which contributes to feeling satisfied after smaller meals. This dual action may provide more significant appetite suppression than single-mechanism medications. Mounjaro comes as a pre-filled KwikPen and is injected once weekly under the skin on your stomach, upper thigh, or upper arm. Doses start at 2.5mg and can increase up to 15mg based on individual response and tolerability.
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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.
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Mounjaro Jab Eligibility and Access in the UK
You may be eligible for a Mounjaro jab in the UK 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, or cardiovascular disease. Use our free BMI calculator above to check your personal BMI. Lower thresholds apply for adults of South Asian, Chinese, Black African, or Black Caribbean heritage (from BMI 27.5). Access is available through NHS specialist tier 3/4 weight management services (via GP referral) or through private prescribing following an online clinical assessment with a UK-licensed prescriber. NHS waiting lists vary significantly by region. Private access is typically faster but involves costs ranging from approximately £150–£320 per month depending on your dose.
- 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.
How much weight can I expect to lose with a Mounjaro jab?
Clinical trials (SURMOUNT-1) showed average weight loss of 15–21% of body weight over 72 weeks at the maximum 15mg dose, alongside reduced-calorie diet and exercise. However, individual results vary significantly — some people may lose more, others less. Weight loss depends on your starting weight, adherence to dietary changes, activity level, and how your body responds to the medication.
Is Mounjaro the same as Wegovy?
No. Both are weight loss injections, but Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, while Wegovy (semaglutide) is a single GLP-1 receptor agonist. Mounjaro's dual mechanism may provide stronger appetite suppression. Average weight loss is slightly higher with Mounjaro (15–21%) compared to Wegovy (approximately 15%). Try our GLP-1 weight loss calculator to compare projected results at different doses.
What are the main side effects of Mounjaro jabs?
Very common side effects include nausea, diarrhoea, vomiting, and stomach pain — most mild and temporary as your body adjusts. Common side effects include indigestion, fatigue, dizziness, and injection site reactions. Serious side effects are rare but can include severe abdominal pain (pancreatitis), gallbladder problems, or serious allergic reactions. Always report persistent or severe symptoms to your GP or prescriber.
Can I get Mounjaro on the NHS?
Yes, Mounjaro (tirzepatide) is available on the NHS through specialist tier 3/4 weight management services. You'll need a GP referral. Eligibility and waiting times vary significantly by region. Many areas have long waiting lists. Private prescribing offers faster access through online clinical assessments, though at private cost. Ask your GP about local NHS availability first.
How do I inject a Mounjaro jab?
Mounjaro comes as a pre-filled KwikPen with clear instructions. You inject once weekly, rotating between injection sites: stomach (abdomen), upper thigh, or upper arm. Your prescriber or pharmacist will demonstrate the injection technique. Each pen contains four weekly doses. Store unopened pens in the fridge (2–8°C); after first use, store at room temperature for up to 30 days. Always dispose of needles safely in a sharps bin.
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.