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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Eli Lilly Mounjaro — What You Need to Know
Mounjaro is a prescription-only weight management injection manufactured by Eli Lilly and Company. It contains tirzepatide, a dual GIP and GLP-1 receptor agonist — a different mechanism to other weight loss medications. Mounjaro is licensed in the UK for adults with obesity or overweight with weight-related health conditions. It's administered as a once-weekly injection using a pre-filled KwikPen device. Understanding how Mounjaro works, who can access it, and what to expect is essential before considering this treatment option.
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Calculate Your BMI Instantly
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 Makes It Different
Mounjaro's dual mechanism sets it apart from other weight management injections. It activates two natural hormone pathways: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual action may help regulate appetite, reduce food cravings, and support feeling fuller after smaller meals. The medication also slows gastric emptying — how quickly food leaves the stomach — though this effect typically reduces over time. Unlike insulin, Mounjaro does not work by lowering blood sugar directly. In clinical trials (SURMOUNT-1), participants taking the maximum dose of 15 mg lost an average of approximately 20.9% of their body weight over 72 weeks, alongside a reduced-calorie diet and increased physical activity. Individual results vary significantly. Use our free BMI calculator and GLP-1 weight loss calculator to understand your starting point and project potential outcomes at different doses.
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No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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 How to Access Mounjaro
In the UK, Mounjaro may be prescribed for adults with a BMI of 30 kg/m² or above, or 27–29.9 kg/m² 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 be eligible at a BMI of 27.5 or above due to higher health risks at lower BMI levels. Doses start at 2.5 mg and escalate weekly up to a maximum of 15 mg. Each dose is a once-weekly injection administered at home. Access in the UK is through NHS specialist tier 3/4 weight management services (referral required) or private online prescribing following a clinical assessment. A prescriber will evaluate your medical history and suitability before issuing a prescription.
- 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 is the difference between Mounjaro and Wegovy?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist manufactured by Eli Lilly, whilst Wegovy (semaglutide) is a single GLP-1 agonist made by Novo Nordisk. Clinical trials showed Mounjaro achieved average weight loss of approximately 20.9% body weight, whilst Wegovy achieved approximately 14.9%. Both are prescription-only and require clinical assessment. Individual results vary considerably.
How long does it take to see weight loss results with Mounjaro?
Weight loss typically begins within the first few weeks, though most significant results are seen after 12–16 weeks of consistent use at a stable dose. Clinical trial data was collected over 72 weeks. Mounjaro works best alongside a reduced-calorie diet and increased physical activity. Your GP or prescriber can discuss realistic timelines for your situation.
What are the most common side effects of Mounjaro?
Very common side effects include nausea, diarrhoea, vomiting, constipation, and stomach pain. These often improve as your body adjusts. Less common but serious side effects include severe stomach pain (possible pancreatitis), gallbladder problems, and serious allergic reactions. Always report persistent or severe symptoms to your prescriber immediately.
Can I get Mounjaro on the NHS in the UK?
Mounjaro is available through NHS specialist weight management services (tier 3/4) in some regions, but availability and waiting times vary significantly by area. Many people access it privately through UK-licensed prescribers following an online consultation and clinical assessment. A private prescription typically costs £129–£320 monthly depending on dose.
Is Mounjaro suitable if I have type 2 diabetes?
Yes — type 2 diabetes is one of the weight-related conditions that may make you eligible for Mounjaro. However, if you take diabetes medications like insulin or sulphonylureas, your doses may need adjustment when starting Mounjaro. Always inform your prescriber of all current medications, and they will manage any necessary changes safely.
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.