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 Mounjaro — UK Injection Instructions
Mounjaro (tirzepatide) is a prescription-only weight loss injection that comes as a pre-filled KwikPen device, designed for once-weekly self-administration at home. This guide explains how to inject Mounjaro safely in the UK, including preparation, injection technique, storage requirements, and what to do if you miss a dose. Always follow your prescriber's instructions and consult your GP if you have any concerns about using your Mounjaro KwikPen.
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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 Injection Instructions
Before injecting: Wash your hands thoroughly with soap and water. Remove your Mounjaro KwikPen from the refrigerator and let it sit at room temperature (below 30°C) for 15–30 minutes — do not warm it artificially. Check that the liquid inside is clear and colourless; if it appears cloudy or discoloured, do not use it.
Preparing the injection site: Choose one of three areas: your stomach (abdomen), upper thigh, or upper arm (upper arm requires assistance from another person). Rotate your injection site each week to prevent lipohypertrophy (thickened skin). Clean the area with an alcohol wipe and allow it to dry completely before injecting.
Injecting: Hold the KwikPen firmly and push the needle straight into your skin at a 90-degree angle. Press the button and hold for 3 seconds to ensure the full dose is delivered. Remove the needle and dispose of it safely in a sharps bin. Do not recap the needle.
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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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Storage, Missed Doses, and Common Questions
Storage requirements: Unopened Mounjaro pens must be stored in a refrigerator at 2–8°C in their original carton. Do not freeze them — if frozen, they cannot be used. After first use, your pen can be kept at room temperature (below 30°C) for up to 30 days. Dispose of the pen 30 days after first injection, even if medication remains.
If you miss a dose: If you remember within 4 days of your scheduled injection, inject as soon as possible and continue your normal weekly schedule. If more than 4 days have passed, skip that dose and inject on your next scheduled day. Never inject a double dose to make up for a missed one. If you're unsure, contact your prescriber or pharmacist for guidance.
- 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.
What size is the Mounjaro needle?
Mounjaro KwikPen needles are 31 gauge, which are very thin and relatively painless. The needle length is 6mm or 8mm depending on which needle you are using. Most people experience minimal discomfort. If you're nervous about injections, your GP or pharmacist can talk you through the technique.
Can I reuse my Mounjaro needle?
No — each Mounjaro needle should be used once only and then safely disposed of in a sharps bin. Reusing needles increases infection risk and the needle becomes blunt, making injection more painful and potentially ineffective.
What happens if I inject Mounjaro into the wrong area?
Mounjaro is designed for subcutaneous injection (under the skin) in the stomach, thigh, or upper arm. Injecting into muscle or vein by accident is unlikely if you follow instructions, but would require immediate medical attention. Always follow the approved injection sites listed in your patient information leaflet.
Can I use my Mounjaro if it's been frozen?
No — if your Mounjaro pen has been frozen, do not use it. Freezing damages the medication. If this happens, contact your pharmacist or prescriber for a replacement. Always store unopened pens in the refrigerator (2–8°C), never in the freezer compartment.
How do I dispose of my used Mounjaro pen safely?
Place used needles and empty pens in a sharps bin (a yellow plastic container available from your GP surgery or pharmacist). Never put Mounjaro needles or pens in household waste or recycling bins. If you don't have a sharps bin at home, ask your pharmacist for one — they're provided free of charge in the UK.
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