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
Free tracker included with every download this week
How to Prepare Your Mounjaro Pen Correctly
Preparing your Mounjaro KwikPen correctly before each injection is essential for safety and effectiveness. This guide walks you through every step of getting your tirzepatide pen ready, from storage checks to final preparation, using UK healthcare standards and MHRA guidance. Whether you're new to Mounjaro or want to refresh your technique, following these steps ensures you're administering your prescription medication safely and confidently each week.
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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.
Step-by-Step Mounjaro Pen Preparation Guide
Before your injection day: Check that your Mounjaro KwikPen has been stored correctly in a refrigerator at 2–8°C if unopened, or at room temperature below 30°C if already in use (for up to 30 days). Remove the pen from the fridge 15 minutes before use to allow it to reach room temperature — injecting a cold medication can cause discomfort.
On injection day: Wash your hands thoroughly with soap and water. Inspect the KwikPen for any visible damage, cracks, or leakage — do not use if damaged. Check the expiry date on the label. Look at the medication through the viewing window: it should be clear and colourless. If it appears cloudy, discoloured, or contains particles, discard it and use a new pen.
- Remove the grey base cap by twisting it off
- Remove the needle safety cap (yellow cap)
- Wipe the injection site on your abdomen, thigh, or upper arm with an alcohol wipe and let it air dry
- Your pen is now ready to inject
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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 and Safety Checks Before Each Use
Unopened Mounjaro pens must be stored in the refrigerator at 2–8°C in their original carton to protect from light — never freeze them. If a pen has been frozen, do not use it. Once you start using a pen, you can keep it at room temperature (below 30°C) for up to 30 days; after 30 days, dispose of it safely even if medication remains.
Always check the temperature before injection. If your pen feels too cold, leaving it on a clean surface for 15 minutes allows it to warm naturally. Never use a microwave, hot water, or direct heat. If you're unsure whether your pen is safe to use — if it looks damaged, has been exposed to extreme temperatures, or if you've missed a dose and it's been longer than 4 days — contact your prescriber or pharmacist. Your GP or pharmacy can advise whether you should continue with that pen or request a replacement.
- 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 use my Mounjaro pen straight from the fridge?
It's better to let your pen reach room temperature first. Remove it from the fridge 15 minutes before injection. Injecting cold medication can cause discomfort and may affect absorption. Room temperature preparation is gentler on the injection site and more comfortable for most patients.
What should I do if my Mounjaro pen looks cloudy?
Do not use it. Mounjaro should be clear and colourless. Cloudy, discoloured, or particles in the medication indicate the pen is damaged or contaminated. Dispose of it safely and contact your pharmacy to request a replacement pen before your next scheduled injection.
How long can I keep Mounjaro at room temperature after opening?
Up to 30 days after your first injection. After 30 days, discard the pen even if medication remains. Keep it below 30°C, away from heat and direct sunlight. If you're unsure how long your pen has been at room temperature, it's safer to use a new one.
Can I inject Mounjaro through clothing?
No. Always inject into bare skin on your abdomen, upper thigh, or upper arm. Injecting through fabric prevents you from seeing the injection site and may affect the needle's ability to reach the subcutaneous tissue correctly. Prepare your skin with an alcohol wipe and allow it to dry.
What happens if I damage my Mounjaro pen before injection?
Do not use a damaged pen. Visible cracks, leaks, or damage to the KwikPen means the medication may be contaminated or the dose may be incorrect. Contact your prescriber or pharmacy immediately to arrange a replacement. Never attempt to repair or reuse a damaged pen.
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.