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 Give Mounjaro Injections Safely
If you've been prescribed Mounjaro (tirzepatide) for weight management in the UK, you'll be self-administering once-weekly injections at home using a pre-filled KwikPen device. This guide walks you through the exact steps to give your Mounjaro injection safely and confidently. Proper injection technique ensures you receive the full dose and minimises discomfort. Your prescriber and the patient information leaflet will also provide specific guidance tailored to your prescription.
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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 Technique
Before you inject: Wash your hands thoroughly with soap and water. Remove your Mounjaro KwikPen from the refrigerator (if stored cold) and allow it to reach room temperature for at least 15 minutes — never inject a cold injection. Check the pen is not leaking and the liquid inside is clear and colourless. If the liquid is cloudy, discoloured, or contains particles, do not use it.
Choosing your injection site: Mounjaro can be injected into your stomach (abdomen), upper thigh, or upper arm. Rotate your injection site each week to avoid lipodystrophy (changes in fat tissue). Avoid areas that are tender, bruised, hard, or scarred. Clean the injection site with an alcohol wipe and allow it to dry completely (approximately 10 seconds) before injecting.
Giving the injection: Remove the needle cap by pulling it straight off. Hold the pen firmly at a 90-degree angle (straight out from your skin). Press the injection button slowly until you hear or feel a click, then hold in place for 3 seconds to ensure the full dose is delivered. Withdraw the needle straight out and apply gentle pressure with a clean swab for 5–10 seconds if bleeding occurs.
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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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After Your Injection And Storage Tips
After injecting, dispose of your used KwikPen safely in a sharps bin — never put it in household waste or recycling. If you don't have a sharps bin, ask your pharmacist for one. Apply a plaster if needed. You may experience mild injection site reactions such as redness, bruising, or tenderness — these usually resolve within a few hours to days.
Storage: Unopened pens must be stored in a refrigerator at 2–8°C in the original carton. Do not freeze. After first use, your pen can be kept at room temperature (below 30°C) for up to 30 days, then discarded. Mark the date of first use on your pen. If you forget a dose, inject as soon as you remember if it's within 4 days. If more than 4 days have passed, skip that dose and take your next injection on the normally scheduled day. Always keep your next dose at least 72 hours apart.
- 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 myself at home?
Yes. Once you've received training from your healthcare provider or pharmacist, you can self-administer Mounjaro at home using the pre-filled KwikPen. Many people find home injection more convenient than clinic visits. Always follow the instructions provided with your specific prescription and ask your prescriber if you're unsure about any step.
Which injection sites can I use for Mounjaro?
You can inject Mounjaro into your stomach (abdomen), outer upper thigh, or upper arm. The stomach is the most common site. Rotate your injection site each week to prevent lipodystrophy (changes in fat tissue). Avoid areas that are tender, bruised, hard, scarred, or where you have psoriasis or eczema.
What should I do if I miss a Mounjaro dose?
If you remember within 4 days of your missed injection, take it as soon as you remember, then continue with your normal schedule the following week. If more than 4 days have passed, skip that dose and inject on your normally scheduled day. Never take a double dose. Ensure at least 72 hours between injections.
Does Mounjaro need to be refrigerated at home?
Unopened Mounjaro pens must be stored in a refrigerator at 2–8°C in the original carton. After your first injection, you can store the pen at room temperature (below 30°C) for up to 30 days. Never freeze Mounjaro — if frozen, do not use it. Discard the pen 30 days after first use, even if it contains remaining medication.
What are common injection site side effects with Mounjaro?
Mild redness, bruising, swelling, or tenderness at the injection site are common and usually resolve within hours to days. These typically improve as your body adjusts. If injection site reactions are severe, persistent, or spreading, contact your GP or prescriber. Rotating your injection site each week can help reduce these reactions.
Ready to take control of your weight loss journey?
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