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 — Complete UK Instructions
Mounjaro (tirzepatide) is a prescription-only, once-weekly weight loss injection that comes in a pre-filled KwikPen device. If you've been prescribed Mounjaro as part of your weight management plan, learning the correct injection technique is essential for getting the most from your treatment. This guide walks you through the step-by-step process of preparing and administering your weekly Mounjaro injection, storage requirements, choosing injection sites, and what to do if you miss a dose. Always follow your prescriber's guidance and the instructions in your medicine information leaflet.
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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 you inject: Remove your KwikPen from the fridge 15 minutes before injection to let it reach room temperature — cold injections may be uncomfortable. Check the medicine inside the pen through the window; it should be clear and colourless. If it looks cloudy, discoloured, or contains particles, do not use it.
Preparing the injection site: Choose your stomach (abdomen), upper thigh, or upper arm. Rotate injection sites each week to prevent skin irritation. Clean the injection site with an alcohol swab and let it dry completely.
Injecting: Pinch the skin gently, hold the KwikPen at 90 degrees against your skin, and press the injection button firmly until you hear a click. Keep holding for 3 seconds after the click, then remove the pen. A small amount of blood or clear fluid is normal. Do not rub the injection site — this can affect how the medicine is absorbed.
After injection: Place the used pen in a sharps bin or return it to your pharmacy. Do not throw it in household waste.
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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 Important Tips
Storage before first use: Keep unopened Mounjaro pens in the fridge between 2°C and 8°C in the original carton. Do not freeze — if frozen, the pen cannot be used. Check the expiry date before using.
Storage after first use: Once you've used a pen for the first time, you can keep it at room temperature (below 30°C) for up to 30 days. After 30 days, dispose of it safely in a sharps bin, even if medicine remains.
If you miss a dose: If you remember within 4 days of your scheduled injection, take it as soon as you remember and continue your normal weekly schedule. If more than 4 days have passed, skip that dose and inject on your normal scheduled day. Never take two doses within 3 days of each other.
Side effects during injection: Mild redness, bruising, or soreness at the injection site is common and usually resolves within a few hours. If you experience severe pain, swelling, or signs of infection, contact your GP.
- 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?
Yes, you can inject into your upper arm, but it may be more comfortable to have someone else do it for you since the angle can be difficult to manage alone. Your stomach (abdomen) and upper thigh are easier for self-injection. Always ask your prescriber if you're unsure.
What should I do if I inject Mounjaro but nothing comes out?
If you press the button but don't hear a click or see the dose counter move, the injection may not have worked. Do not inject again with the same pen. Contact your prescriber or pharmacy immediately for guidance — you may need to use a new pen for your dose.
Can I reuse a Mounjaro pen?
No. Each KwikPen is single-use only. Once you've completed all 4 doses in a pen (4 weeks of injections), dispose of it safely in a sharps bin and use a new pen. Never attempt to refill a pen.
Is there a best time of day to inject Mounjaro?
Mounjaro can be injected at any time of day that suits you. Choose a day and time you'll remember each week — consistency helps you build a routine. You can change your injection day if needed, but maintain at least 3 days between doses.
What's the difference between Mounjaro and Wegovy injection technique?
Both use pre-filled pens and are injected once weekly under the skin. The main differences are the medication itself (Mounjaro is a dual GIP/GLP-1 agonist; Wegovy is a GLP-1 agonist only) and starting doses. Injection technique is essentially the same. Your prescriber will give you specific guidance for whichever medication you've been prescribed.
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