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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Mounjaro Diet Injection — How It Works for Weight Management
Mounjaro is a prescription-only weekly weight loss injection licensed in the UK for adults with obesity or overweight with health complications. It contains tirzepatide, a dual GLP-1 and GIP receptor agonist that works by reducing appetite and helping you feel fuller after smaller meals. Unlike some diet trends, Mounjaro is an evidence-based medication backed by clinical trials. It's used alongside a reduced-calorie diet and increased physical activity — not instead of lifestyle changes. This guide explains how Mounjaro works, who can access it, 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 as a Diet Injection
Mounjaro is a dual mechanism weight loss injection that activates two natural hormone pathways: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual action is what distinguishes it from other weekly weight loss injections like Wegovy, which targets GLP-1 alone.
When you inject Mounjaro once weekly using the pre-filled KwikPen device, it signals your brain that you're satisfied after eating smaller portions, reduces food cravings, and slows how quickly food moves through your stomach. This combination may help you consume fewer calories naturally without feeling deprived. In the SURMOUNT clinical trials, participants on Mounjaro lost an average of 15–21% of their body weight over 72 weeks, though individual results vary significantly. The medication works best when combined with a reduced-calorie diet and regular physical activity — it's a tool to support these efforts, not a replacement for them.
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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 Side Effects in the UK
You may be eligible for Mounjaro in the UK if you have a BMI of 30 or above, or BMI of 27–29.9 with weight-related health conditions such as type 2 diabetes, prediabetes, high blood pressure, or cardiovascular disease. Adults of South Asian, Chinese, Black African, or Black Caribbean heritage may have lower BMI thresholds due to higher health risks at lower weights. Mounjaro starts at 2.5 mg weekly and increases gradually to a maximum of 15 mg — each dose adjustment requires at least 4 weeks. Common side effects include nausea, diarrhoea, constipation, and stomach pain, usually mild and temporary. Serious side effects like severe abdominal pain require immediate medical attention. You can access Mounjaro through NHS specialist weight management services or private online prescribing following a clinical assessment.
- 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?
Yes, NICE has approved tirzepatide (Mounjaro) for use within NHS specialist weight management services. However, access is through tier 3 specialist services, not standard GP prescription. Availability varies significantly by region and waiting lists can be several months to years. Private access is available through GPhC-registered online pharmacies following a clinical assessment.
How much weight will I lose with Mounjaro?
In clinical trials, participants lost an average of 15–21% of their body weight over 72 weeks on the maximum dose, alongside diet and exercise. However, individual results vary considerably — some people lose more, others less. Weight loss depends on dose, adherence, diet changes, physical activity, and individual metabolism. Your prescriber can discuss realistic expectations for your situation.
What's the difference between Mounjaro and Wegovy as diet injections?
Both are once-weekly weight loss injections, but Mounjaro (tirzepatide) is a dual GIP+GLP-1 agonist, while Wegovy (semaglutide) targets GLP-1 alone. Mounjaro may offer greater weight loss potential in clinical trials. Both require prescription and clinical assessment. The choice depends on individual factors and prescriber recommendation — our GLP-1 weight loss calculator can help you understand potential outcomes at different doses.
Can I use Mounjaro without changing my diet?
Mounjaro works best when combined with a reduced-calorie diet and increased physical activity. While the injection reduces appetite, making healthier food choices and regular exercise significantly improve results and overall health outcomes. Your prescriber or a dietitian can help create a sustainable plan alongside Mounjaro.
What happens if I stop using Mounjaro?
If you stop Mounjaro, appetite-suppressing effects wear off within days to weeks, and weight may be regained if diet and exercise habits aren't maintained long-term. Weight management is typically ongoing — Mounjaro is a support tool, not a permanent supports. Discuss long-term plans with your prescriber.
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.