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 Patient Information Guide for UK Users
Mounjaro (tirzepatide) is a prescription-only weight management injection licensed in the UK for adults with excess weight. This guide provides essential patient information covering how Mounjaro works, dosing schedules, potential side effects, storage requirements, and eligibility criteria. If you're considering this medication, understanding the full patient information is vital before speaking with your GP or licensed prescriber. Use our free BMI calculator and GLP-1 weight loss calculator above to assess whether you might be eligible and to estimate potential weight loss outcomes.
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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 Dosing Schedule
Mounjaro contains tirzepatide, a dual GIP and GLP-1 receptor agonist that works differently from other weight loss medications. It activates two natural hormone pathways that help regulate appetite, reduce food cravings, and support feeling fuller after meals. Mounjaro is injected once weekly using a pre-filled KwikPen device.
Standard dosing progression:
- Weeks 1–4: 2.5 mg (starting dose)
- Weeks 5–8: 5 mg (first maintenance dose)
- Weeks 9–12: 7.5 mg (if escalation needed)
- Weeks 13+: 10 mg, 12.5 mg, or 15 mg (maximum dose)
Each dose is held for a minimum of 4 weeks before increasing. The maximum recommended dose is 15 mg once weekly. Clinical trials showed average weight loss of approximately 15–21% of body weight, though individual results vary significantly and depend on dose, diet, and physical activity.
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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.
Side Effects, Eligibility, and Safe Use
Common side effects include nausea, diarrhoea, constipation, and stomach pain—typically mild and improving over time. Less common side effects include vomiting, indigestion, fatigue, and injection site reactions. Seek urgent medical attention for severe persistent stomach pain (possible pancreatitis), allergic reactions, or gallbladder problems.
UK eligibility: Adults with BMI 30 or above, or BMI 27–29.9 with weight-related health conditions (diabetes, high blood pressure, sleep apnoea, cardiovascular disease). South Asian, Chinese, Black African, and Black Caribbean adults may qualify at lower BMI thresholds. Mounjaro is not suitable during pregnancy, whilst breastfeeding, or for under-18s. Always consult your GP or licensed prescriber to confirm suitability.
- 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 do I inject Mounjaro using the KwikPen?
Mounjaro is injected once weekly into your stomach, upper thigh, or upper arm using a pre-filled KwikPen. Rotate injection sites each week. Inject slowly and steadily, holding the pen in place for 5–10 seconds. Always use a new needle for each injection. Your prescriber or pharmacist will provide full injection training before you start.
Can I switch between Mounjaro and Wegovy?
Mounjaro (tirzepatide) and Wegovy (semaglutide) are different medications with different mechanisms. Mounjaro is a dual GIP/GLP-1 agonist, whilst Wegovy is a single GLP-1 agonist. Approximate dose equivalence exists, but any switch must be managed by your prescriber. Do not change medications without medical guidance.
How should I store my Mounjaro KwikPen?
Store unopened pens in a refrigerator at 2–8°C in the original carton. Do not freeze. After first use, you can store the pen at room temperature (below 30°C) for up to 30 days, then dispose of it. Keep away from heat and direct sunlight at all times.
What happens if I miss a dose of Mounjaro?
If you miss a dose within 4 days, inject as soon as remembered and continue your normal schedule. If more than 4 days have passed, skip that dose and take your next dose on the regular day. Never double-dose. The minimum interval between doses is 72 hours (3 days).
Does Mounjaro interact with other medications I'm taking?
Mounjaro may affect how your body absorbs oral medications, including oral contraceptives. Women taking the contraceptive pill should use barrier contraception or switch methods for 4 weeks after starting Mounjaro and 4 weeks after each dose increase. Inform your prescriber of all medications you're taking before starting treatment.
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.