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 Inject Your Mounjaro Pen Safely
If you've been prescribed Mounjaro (tirzepatide) in the UK, you'll receive a pre-filled KwikPen device that you inject once weekly under the skin. Injecting Mounjaro is straightforward when you follow the correct technique — this guide walks you through each step, from checking your pen to disposing of it safely. Always follow the specific instructions provided with your prescription, and ask your prescriber or pharmacist if you're unsure about any part of the process.
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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.
Step-By-Step Mounjaro Injection Instructions
Before you inject: Remove your Mounjaro KwikPen from the fridge 15 minutes before use — it works best at room temperature. Check the liquid is clear and colourless; if it looks cloudy or discoloured, do not use it. Wash your hands with soap and water. Choose your injection site: stomach (abdomen), upper thigh, or upper arm. Rotate your site each week to prevent lipodystrophy (skin thickening). If using your stomach or thigh, you can inject yourself. If using your upper arm, ask someone to help you.
Injecting: Pinch the skin gently at your chosen site. Holding the pen firmly, push the needle straight into your skin at a 90-degree angle. Press and hold the grey button until you hear two clicks (this takes about 6 seconds). Keep holding for 6 more seconds after the second click. Slowly pull the needle out. You don't need to rub the injection site — a plaster is optional.
After injection: Replace the grey needle cover and dispose of your used pen safely in a sharps bin (ask your GP or pharmacist for a free sharps container). Record your injection date so you remember when to take your next dose.
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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.
Storage, Missed Doses, and When To Seek Help
Storage: Unopened pens stay in the fridge at 2–8°C in their original carton. Once you start using a pen, you can keep it at room temperature (below 30°C) for up to 30 days. Never freeze your Mounjaro — if it freezes, discard it. Each KwikPen contains 4 weekly doses.
Missed dose: If you forget your injection, take it within 4 days of your usual day — then continue on your normal schedule the following week. If more than 4 days have passed, skip that dose and inject on your next scheduled day. Never double-dose.
Seek medical help immediately if: You have severe stomach pain spreading to your back (possible pancreatitis), serious allergic reaction (swelling of face or throat), or changes in vision. Contact your GP or NHS 111 for advice on any side effects or concerns about your injections.
- 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.
Can I inject Mounjaro myself, or do I need help from my GP?
Most people can inject Mounjaro themselves once trained. You can inject your stomach or thigh independently. If using your upper arm, ask someone to help. Your prescriber or pharmacist will show you the correct technique before your first dose. If you're unsure, your GP practice can administer injections for you.
What if I accidentally inject into a blood vessel?
Injecting into a blood vessel is unlikely with proper technique — you're injecting under the skin, not into deep tissue. If you think you've hit a vessel (blood comes out), don't worry; it's not dangerous. Simply apply gentle pressure with a cotton ball. Contact your pharmacist if you're concerned about future injections.
Does Mounjaro injection hurt?
Most people report minimal discomfort — the needle is very fine. You may feel a small pinch. Some people experience mild stinging or bruising at the injection site, which usually settles within hours. Rotating your injection site each week helps reduce irritation and bruising over time.
Can I use a Mounjaro pen that's been left at room temperature?
Yes, once you've started using a pen, you can keep it at room temperature (below 30°C) for up to 30 days. However, unopened pens must stay refrigerated. If a pen has been left out of the fridge unopened for more than a few hours, ask your pharmacist whether it's safe to use.
What should I do if I get a side effect like nausea after injecting?
Nausea is very common, especially in the first few weeks. Take your time eating — smaller, more frequent meals and avoiding fatty foods often helps. Stay hydrated. Most nausea improves as your body adjusts to the dose. If nausea is severe or persistent, contact your GP or pharmacist; they may suggest adjusting your dose or provide anti-nausea advice.
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.