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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How to Use Mounjaro: A Complete UK Guide
Mounjaro (tirzepatide) is a prescription-only, once-weekly injectable medication used for weight management in UK adults. If your GP or prescriber has prescribed Mounjaro, understanding how to use it correctly — including injection technique, storage, dose timing, and what to expect — is essential for safe and effective treatment. This guide covers everything you need to know about using Mounjaro as part of your weight management journey alongside lifestyle changes.
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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 is a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways that help regulate appetite and support feeling fuller after meals. Each dose comes in a pre-filled KwikPen device containing enough medication for one week.
Treatment starts at 2.5 mg weekly, with doses gradually increasing every 4 weeks: 5 mg, 7.5 mg, 10 mg, 12.5 mg, and up to a maximum of 15 mg weekly. Your prescriber will determine your starting dose and when to increase based on your individual response. Clinical trials showed that higher doses (10–15 mg) were associated with greater average weight loss, though individual results vary significantly. Each KwikPen contains 4 weekly doses, so one pen lasts approximately one month.
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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.
Injection Technique and Storage
Mounjaro is injected once weekly into the stomach (abdomen), upper thigh, or upper arm — rotate your injection site each week to prevent skin irritation. The KwikPen is simple to use: check the medication is clear, hold the pen at 90 degrees to your skin, press the button firmly, and count to 3 before removing the needle. Store unopened pens in a refrigerator at 2–8°C and keep them in their original carton. Once you start using a pen, it can be kept at room temperature (below 30°C) for up to 30 days. Never freeze Mounjaro — if frozen, discard it. Dispose of used pens safely in a sharps container or ask your pharmacy for disposal guidance.
- 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 should I do if I miss a weekly Mounjaro injection?
If you miss a dose within 4 days, take it as soon as you remember and continue your normal weekly schedule. If more than 4 days have passed, skip that dose and take your next injection on the scheduled day. Never take a double dose. Minimum time between doses is 72 hours.
Can I switch between injection sites, and does it matter which one I use?
Yes, you should rotate sites weekly to prevent local skin reactions. The stomach, upper thigh, and upper arm are all suitable. Rotating sites helps prevent bruising, redness, and irritation. Always allow the injection area to fully recover before injecting there again.
Will Mounjaro cause side effects, and what should I expect?
Common side effects include nausea, diarrhoea, constipation, and stomach pain — these often improve as your body adjusts. Nausea is most common when starting or increasing doses. If side effects are severe or persistent, contact your prescriber. Serious side effects requiring urgent medical attention include severe stomach pain (possible pancreatitis) or serious allergic reactions.
How long do I need to use Mounjaro, and what happens if I stop?
Your prescriber will discuss treatment duration with you. Mounjaro is typically used as a long-term weight management tool alongside diet and exercise. If you stop Mounjaro, appetite and food cravings may return. Weight regain is possible after stopping, so maintaining lifestyle changes is important.
Can I use our GLP-1 weight loss calculator to estimate my potential results with Mounjaro?
Yes — our free GLP-1 weight loss calculator can help you see projected weight loss outcomes at different doses, based on clinical trial data. Remember that individual results vary significantly depending on your starting weight, adherence to lifestyle changes, and personal factors. Use the calculator alongside the BMI calculator to track your health metrics.
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.