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 Prescription Mounjaro and How Does It Work?
Mounjaro is a prescription-only weight management injection available in the UK for eligible adults. It contains tirzepatide, a dual GIP and GLP-1 receptor agonist that works by helping to reduce appetite, support feeling fuller after meals, and regulate blood sugar. Unlike some other weight loss medications, Mounjaro activates two natural hormone pathways, which may contribute to its effectiveness. It's administered as a once-weekly injection using a KwikPen device. This guide explains how prescription Mounjaro works, who may be eligible, 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 Prescription Mounjaro Works and Its Eligibility
Mounjaro mechanism: Tirzepatide mimics two natural gut hormones — GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual action may help reduce appetite, lower food cravings, and support feeling satisfied with smaller portions. It also slows gastric emptying, meaning food leaves your stomach more slowly, which contributes to prolonged fullness.
UK prescription eligibility: Adults with a BMI of 30 kg/m² or above may be eligible, or those with a BMI of 27–29.9 kg/m² and at least one weight-related health condition (type 2 diabetes, prediabetes, high blood pressure, abnormal blood fats, sleep apnoea, or cardiovascular disease history). For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, lower BMI thresholds of 27.5+ may apply due to increased health risks at lower weights.
Prescription Mounjaro requires a clinical assessment from a UK-licensed prescriber to confirm suitability. It cannot be taken during pregnancy, breastfeeding, or by anyone under 18.
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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 Doses, Side Effects, and Access Routes
Dose titration: Prescription Mounjaro starts at 2.5 mg weekly and increases over 16+ weeks to a maximum of 15 mg weekly. Your prescriber adjusts the dose based on your response and tolerance. Each KwikPen contains four weekly doses.
Common side effects: Nausea, diarrhoea, vomiting, constipation, and abdominal pain are most frequent, particularly at the start. Injection site reactions, fatigue, dizziness, and hair loss may occur. Most side effects improve over time.
UK access: Prescription Mounjaro is available through NHS specialist weight management services (tier 3/4) or private prescribing following an online assessment. NHS access varies significantly by region and typically involves referral from your GP. Private access is faster but involves a cost. Calculate your potential weight loss using our free GLP-1 weight loss calculator to explore outcomes at different doses.
- 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 prescription Mounjaro and Wegovy?
Both are prescription-only weight management injections licensed in the UK. Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist with a maximum dose of 15 mg weekly. Wegovy (semaglutide) is a GLP-1 agonist only, with a maximum dose of 2.4 mg weekly. Clinical trials suggest Mounjaro may produce slightly greater average weight loss (~15–21% body weight) compared to Wegovy (~15%), though individual results vary significantly.
Can I get Mounjaro on the NHS in the UK?
Yes, Mounjaro is approved for NHS use through specialist weight management services (tier 3 or 4). Access requires referral from your GP to a specialist centre in your area. Availability and waiting times vary significantly by region and NHS Integrated Care Board (ICB). Private prescription is faster if you access it through a licensed online prescriber, though this involves a cost.
What happens if I miss a Mounjaro dose?
If you miss a dose within four days of when you should have taken it, inject as soon as you remember and continue your normal schedule. If more than four days have passed, skip that dose and take your next injection on your normally scheduled day. Never take a double dose, and ensure at least 72 hours (three days) between injections.
Will I lose weight on prescription Mounjaro?
Clinical trials (SURMOUNT-1) showed that on average, participants lost approximately 15–21% of their body weight over 72 weeks whilst using Mounjaro alongside a reduced-calorie diet and increased physical activity. However, individual results vary significantly — some people lose more, some less. Weight loss is not assured and depends on your adherence to lifestyle changes alongside the medication.
What should I do if I experience severe side effects with Mounjaro?
Most side effects are mild and improve within weeks. However, seek immediate medical attention if you experience severe, persistent stomach pain (possible pancreatitis), serious allergic reactions (facial swelling, throat tightness), changes in vision, or severe dehydration. Contact your GP or call NHS 111 for urgent advice. Always report side effects to your prescriber, especially new or worsening symptoms.
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.