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 Administer Your Mounjaro Injection Safely
Mounjaro (tirzepatide) is a once-weekly prescription weight loss injection administered at home using a pre-filled KwikPen device. This informational guide walks through the step-by-step process of preparing and administering your Mounjaro injection safely, including storage requirements, injection site rotation, and what to expect during administration. Always follow your prescriber's instructions and the guidance in your patient information leaflet. If you're unsure about any step, contact your GP or the prescriber who issued your prescription before injecting.
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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 Injection Administration Process
Before you inject: Remove your Mounjaro KwikPen from the refrigerator and allow it to sit at room temperature (below 30°C) for 15 minutes — do not heat it. Check the label to confirm you have the correct dose, and inspect the pen for damage or discolouration of the liquid. If anything looks unusual, do not use it.
Preparing the injection site: Choose from your stomach (abdomen), upper thigh, or upper arm. Rotate your injection site each week to help prevent lipohypertrophy (thickened skin at repeated sites). Clean the skin with an alcohol swab and allow it to dry completely before injecting.
Administering the injection: Hold the KwikPen firmly against your skin at a 90-degree angle. Press the button on top of the pen slowly until you hear a click, then continue holding for 3 seconds. Withdraw the needle and dispose of it safely in a sharps bin. Do not rub the injection site — gentle pressure is acceptable.
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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.
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Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Storage, Disposal, and Important Safety Points
Storage before first use: Keep unopened Mounjaro pens in the refrigerator at 2–8°C in their original carton. Do not freeze. Each KwikPen contains 4 doses (4 weeks' supply).
Storage after first use: Once you've used your first dose, you can store the pen at room temperature (below 30°C) for up to 30 days. After 30 days, dispose of the pen safely in a sharps bin, even if it contains unused doses.
Missed injection: If you miss a dose, inject within 4 days of your scheduled injection day and continue your normal schedule. If more than 4 days have passed, skip that dose and inject on your next scheduled day. Never take a double dose. Maintain at least 3 days between 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 while it's still cold from the fridge?
No. Remove the KwikPen from the refrigerator and let it reach room temperature for 15 minutes before injecting. A very cold injection may be more uncomfortable. Check that the liquid inside is clear and colourless before use.
What should I do if I accidentally inject into a vein or artery?
Contact your GP or NHS 111 immediately. Although Mounjaro is designed for subcutaneous (under the skin) injection, if you're uncertain about whether you've injected correctly, seek medical advice promptly rather than wait to see if symptoms develop.
Can someone else administer my Mounjaro injection for me?
Yes. If you find self-injection difficult, a family member or carer can administer the injection after being shown the correct technique. The injection site can be the stomach, upper thigh, or upper arm — the upper arm requires assistance from another person.
How long does a Mounjaro injection take to administer?
The actual injection takes only a few seconds once the needle is inserted. Preparation (checking the pen, cleaning the site, allowing it to warm to room temperature) typically takes 5–10 minutes total.
What should I do if I drop or damage my Mounjaro KwikPen?
Do not use a damaged pen. Contact your pharmacy or prescriber to arrange a replacement. Store your backup pens carefully and keep them in their original cartons to prevent damage. Check pens visually before each use.
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.