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: A Step-by-Step UK Guide
Mounjaro (tirzepatide) is a once-weekly prescription injection you can administer at home following proper training from your healthcare provider or pharmacist. This guide walks you through the process, from preparation through disposal, aligned with NHS and MHRA guidance for UK patients. Before your first injection, your prescriber or pharmacist will demonstrate the technique in person — this guide is informational support, not a substitute for that training.
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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.
How to Prepare and Inject Your Mounjaro KwikPen
Before you start: Wash your hands thoroughly with soap and warm water. Remove your Mounjaro KwikPen from the refrigerator (if unopened) and allow it to reach room temperature for 15–20 minutes — injecting cold medication can be uncomfortable. Check the solution through the window at the bottom of the pen: it should be clear and colourless. Do not use if it appears cloudy, discoloured, or contains particles.
Choosing your injection site: Mounjaro is injected under the skin (subcutaneously) in the stomach area (most common), upper thigh, or upper arm. Rotate your injection site each week to prevent skin irritation and lipohypertrophy (thickened skin patches). Mark on a calendar or use our free progress tracker to remember which site you used.
Preparing the site: Clean the chosen injection area with an alcohol-based wipe or cotton ball soaked in rubbing alcohol. Allow the skin to air dry completely — injecting through wet alcohol can cause discomfort and reduce absorption. Do not touch the cleaned area before injecting.
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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.
The Injection Technique and Aftercare
Holding the pen: Remove the needle cap by pulling straight off. Hold the KwikPen firmly in one hand at a slight angle (approximately 45 degrees) against the cleaned skin. With your other hand, pinch a fold of skin gently between your thumb and forefinger to create a raised area.
Injecting: Push the needle through the pinched skin fold firmly and steadily. You will feel a click as the injection starts automatically — hold the pen in place for 3 seconds after the click to ensure the full dose is delivered. You may see a second click after about 6 seconds, indicating injection completion. Withdraw the needle at the same angle you inserted it.
After injection: Do not rub the injection site. A small amount of bleeding or bruising is normal. Apply gentle pressure with a clean tissue if needed. You can resume normal activities immediately.
- 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.
Where should I store my Mounjaro KwikPen when not in use?
Unopened pens should be stored in a refrigerator at 2–8°C, kept in the original carton to protect from light. Do not freeze — if frozen, discard the pen. Once you've used a pen for the first time, it can be stored at room temperature (below 30°C) for up to 30 days. Always check the expiry date printed on the carton.
What should I do if I accidentally inject into a vein or artery?
This is very rare, but if you suspect you've injected into a blood vessel (such as seeing blood immediately), contact your GP or call NHS 111 for advice. Mounjaro is designed for subcutaneous (under-skin) injection only. Proper technique and site rotation minimise this risk significantly.
Can I reuse a Mounjaro needle?
No. Each KwikPen has a single-use needle integrated into the device. Never attempt to reuse the needle. After each injection, dispose of your used pen in a sharps bin provided by your pharmacy or GP surgery — never put it in regular household waste.
How do I know the full dose has been delivered?
You will hear two clicks: one when the injection starts and one when it finishes (after approximately 6 seconds). Keep the pen against your skin for at least 3 seconds after the first click to ensure complete delivery. If you're unsure, contact your prescriber or pharmacist.
What if I miss a weekly injection — how long can I wait before my next dose?
If you're within 4 days of your scheduled injection date, take it as soon as you remember and continue your normal weekly schedule. If more than 4 days have passed, skip the missed dose and inject on your normally scheduled day. Never take a double dose or inject sooner than 3 days after your last injection.
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.