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 Take The Mounjaro Injection Correctly
Taking Mounjaro (tirzepatide) correctly ensures you get the full benefit from this weekly weight loss injection. This guide walks you through the exact steps for preparing, storing, and administering your Mounjaro KwikPen, plus what to do if you miss a dose. As a prescription-only medicine licensed in the UK, Mounjaro must be injected subcutaneously (under the skin) once per week. Always follow your prescriber's instructions and the patient information leaflet that comes with your KwikPen.
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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.
Storing Your Mounjaro KwikPen Safely
Before first use: Keep your unopened Mounjaro pens in a refrigerator at 2°C to 8°C. Store them in the original carton to protect from light and heat. Do not freeze — if a pen has been frozen, discard it and do not use. Your pens are safe to use until the expiry date printed on the carton if kept refrigerated.
After first injection: Once you have used a pen for the first time, you can store it at room temperature below 30°C for up to 30 days. Write the date you first used the pen on the carton so you remember when to discard it. After 30 days, throw the pen away safely in a sharps bin — do not put it in household waste.
Travelling: You can carry your in-use Mounjaro pen outside the fridge for short periods (a few hours). If you are travelling long-distance, ask your prescriber for advice on storage during travel.
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.
Injecting Mounjaro — Step-by-Step Process
Step 1: Remove your KwikPen from storage and let it rest at room temperature for 15 minutes before injecting — do not heat it artificially.
Step 2: Check the liquid inside is clear and colourless. If it looks cloudy, crystallised, or discoloured, do not use it.
Step 3: Wash your hands with soap and water. Choose an injection site on your stomach (abdomen), upper thigh, or upper arm. Rotate your site each week to avoid skin damage.
Step 4: Pinch the skin at your chosen site. Insert the needle at a 90-degree angle (straight in, not at an angle). Press the injection button and hold for 3 seconds, then withdraw the needle. Do not massage the injection site afterwards.
Step 5: Place the used pen in a sharps bin immediately after use.
- 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 into the same spot every week?
No. Rotate your injection site each week to prevent skin damage, lumps, and irritation. You can return to the same area after at least one week, but vary the exact spot within that area. Good rotation helps the medication absorb properly and keeps your skin healthy.
What should I do if I miss a Mounjaro injection?
If you remember within 4 days of your missed dose, inject it as soon as you remember. Continue your normal schedule from the next week. If more than 4 days have passed, skip that dose and inject your next dose on your regular scheduled day. Never take a double dose to make up for a missed one.
Can I use a Mounjaro pen that has been frozen?
No. If your Mounjaro pen has been frozen, discard it immediately and do not use it. Freezing can damage the medication and make it unsafe. If you need a replacement pen, contact your prescriber or pharmacy.
How do I know if my Mounjaro is spoiled or unsafe to use?
Mounjaro should be clear and colourless. Do not use your pen if the liquid looks cloudy, crystallised, discoloured, or contains particles. Also discard any pen that has been dropped or damaged, or any pen kept outside the correct temperature range for too long.
Should I use the GLP-1 weight loss calculator before starting Mounjaro?
Yes. Try our free GLP-1 weight loss calculator to estimate potential weight loss at different doses alongside lifestyle changes. This can help you understand realistic expectations — remember that individual results vary and clinical trials show averages only. Your prescriber will also discuss likely outcomes with you during your assessment.
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.