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 Use A Mounjaro Pen — Complete Injection Guide
Mounjaro (tirzepatide) comes as a pre-filled KwikPen device that you inject once weekly at home. This guide walks you through the exact steps to use your Mounjaro pen safely and correctly. Whether you've just received your first prescription or need a refresher, understanding the injection technique is essential for getting the most from your treatment. Always follow the instructions provided with your specific pen, and speak to your GP or prescriber if you have any concerns about your injection technique.
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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 Pen Injection Instructions
Before you inject: Remove your Mounjaro KwikPen from the refrigerator 15 minutes before use to allow it to reach room temperature. Check that the medication inside looks clear and colourless — do not use if it appears cloudy or contains particles. Wash your hands thoroughly with soap and water. Choose an injection site on your stomach (abdomen), upper thigh, or upper arm. Rotate sites each week to prevent irritation.
- Clean the injection site with an alcohol wipe and let it dry
- Remove the pen cap by pulling it straight off
- Remove the paper tab from the needle
- Hold the pen firmly and push the needle straight into your skin at a 90-degree angle
- Press and hold the grey button until you hear two clicks (takes approximately 3 seconds) — this confirms the full dose has been delivered
- Count to 3 after the second click, then slowly withdraw the needle
- Dispose of the used pen in a sharps bin immediately
Do not recap the needle. Each KwikPen contains one dose and should be discarded after use.
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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.
Mounjaro Storage, Missed Doses, And Common Questions
Storage: Keep unopened pens in the refrigerator at 2–8°C in their original carton. Once you've used a pen, you can store it at room temperature (below 30°C) for up to 30 days. Never freeze Mounjaro — if it freezes, discard it. After 30 days or when empty, dispose of in a sharps bin.
Missed doses: If you forget an injection, take it within 4 days of the missed date and continue your normal schedule. If more than 4 days have passed, skip that dose and inject on your next scheduled day. Maintain at least 72 hours between injections.
Side effects during injection: Mild redness, bruising, or slight pain at the injection site is normal and usually settles within a few hours. Contact your GP if you experience severe pain, swelling, or signs of infection.
- 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 my arm myself?
Injecting into your upper arm is possible but more difficult without assistance due to the angle required. It's generally easier to self-inject into your stomach or thigh. If you choose to use your arm, ask a family member or healthcare professional to assist or demonstrate the technique first.
What size needle does Mounjaro use?
The Mounjaro KwikPen uses a 29-gauge needle (very thin). You don't need to change or handle the needle separately — it's integrated into the pre-filled pen. This makes injection simpler and safer than separate needle-and-syringe systems.
How long does a Mounjaro injection take?
The actual injection takes approximately 3 seconds — you'll hear two clicks as the medication is delivered. The entire process, including preparation and site cleaning, typically takes 2–3 minutes from start to finish.
Can I use a Mounjaro pen that's been frozen?
No. If your Mounjaro has been frozen, discard it and contact your prescriber for a replacement. Freezing can damage the medication and make it unsafe to use, even if it appears normal after thawing.
What should I do if I don't see blood after injecting?
Seeing blood after injection is not necessary and doesn't mean the injection failed. The needle is very thin, and many injections don't produce visible bleeding. If you heard the two clicks and felt the injection, the full dose was delivered correctly.
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.