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 Mounjaro — UK Instructions & Safety Guide
Mounjaro (tirzepatide) is a prescription-only, once-weekly weight loss injection licensed in the UK. If your GP or specialist has prescribed Mounjaro, it's essential to understand the correct injection technique, storage requirements, and what to expect. This guide covers step-by-step instructions for using your KwikPen device, important safety information aligned with NHS guidance, and when to seek medical support. Always follow your prescriber's specific instructions — this information is for educational purposes only.
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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 Technique
Before injecting: Wash your hands thoroughly with soap and water. Remove your Mounjaro KwikPen from the refrigerator and allow it to sit at room temperature for 15–30 minutes — do not heat or microwave it. Check the pen window: the liquid should be clear and colourless. If it appears cloudy, discoloured, or contains particles, do not use it and contact your pharmacist.
Choosing your injection site: Mounjaro can be injected into your stomach (abdomen), upper thigh, or upper arm. Rotate your injection site each week to prevent skin irritation. Avoid areas that are bruised, tender, swollen, or hard. Your thigh or stomach are the easiest sites for self-injection.
The injection itself: Pinch the skin gently around your chosen site. Hold the pen perpendicular (at a 90-degree angle) to your skin. Press the injection button steadily — you should hear or feel a click. Keep the pen pressed against your skin for 3 seconds after the click to ensure the full dose is delivered. Withdraw the pen and release the pinched skin.
After injection: You may see a small drop of blood or fluid — this is normal. You do not need to rub the injection site. If needed, cover with a plaster. Dispose of your pen safely in a sharps bin or as advised by your pharmacist.
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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, Doses, and NHS Access in the UK
Storage before first use: Keep unopened Mounjaro pens in a refrigerator at 2–8°C in their original carton. Do not freeze — if frozen accidentally, discard the pen and obtain a replacement from your pharmacy. Check the expiry date on the carton.
Storage after first use: After your first injection, you can store the pen at room temperature (below 30°C) for up to 30 days. Keep it away from direct sunlight and heat. After 30 days, discard the pen even if it contains unused doses.
NHS prescribing route: Mounjaro is available through NHS specialist weight management services (tier 3) following referral from your GP. A multidisciplinary team will assess your eligibility based on BMI and weight-related health conditions. NICE has approved tirzepatide for NHS use, though availability and waiting times vary significantly by region. You can also access Mounjaro privately through a UK-licensed prescriber following an online clinical assessment.
Mounjaro doses in the UK: Treatment starts at 2.5 mg once weekly, increasing by 2.5 mg every 4 weeks up to a maximum of 15 mg weekly. Your prescriber will advise you on your specific dose schedule. Each KwikPen contains four weekly doses.
What to Expect and When to Seek HelpMost common side effects of Mounjaro include nausea, diarrhoea, constipation, and stomach discomfort — these often improve as your body adjusts. Stay hydrated and eat small, balanced meals. If side effects are severe or persist beyond a few weeks, speak to your GP or prescriber.
Seek urgent medical attention if you experience: Severe, persistent stomach pain spreading to your back (possible pancreatitis), signs of a serious allergic reaction (swelling of face, lips, or throat), significant changes in vision, or severe dehydration. Track your weight loss progress using our free progress tracker to monitor your response to treatment. Our GLP-1 weight loss calculator can also help you understand potential outcomes based on clinical trial data, though individual results vary significantly.
- 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 know if my Mounjaro pen is safe to use?
Check that the liquid inside the pen window is clear and colourless. If it's cloudy, discoloured, or contains particles, do not use it. Always check the expiry date on the carton. If the pen has been frozen, discard it. When in doubt, contact your pharmacist before injecting.
Can I inject Mounjaro in the same spot every week?
No — you should rotate your injection site each week to prevent skin irritation, lumps, and tissue damage. You can use your stomach, thigh, or upper arm. It's safe to return to the same general area after a week or two, but vary the exact spot each time.
What's the difference between NHS and private Mounjaro in the UK?
Both are the same medication. NHS access is through specialist tier 3 weight management services (requires GP referral and has regional waiting lists). Private access is through online prescribers and GPhC-registered pharmacies, available without NHS referral. Both require a clinical assessment; costs and timelines differ.
What should I do if I miss a weekly dose of Mounjaro?
If you remember within 4 days of your missed dose, inject as soon as possible and continue your normal weekly schedule. If more than 4 days have passed, skip that dose and inject on your usual day the following week. Never inject two doses within 3 days. Contact your prescriber if unsure.
Can I store Mounjaro at room temperature before opening it?
No — unopened Mounjaro must be stored in a refrigerator at 2–8°C. Once you've had your first injection from a pen, you can store it at room temperature (below 30°C) for up to 30 days. After 30 days, discard it safely, even if doses remain.
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.