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 Take The Mounjaro Injection Safely
Mounjaro (tirzepatide) is a once-weekly weight loss injection prescribed in the UK for eligible adults. If your GP or a licensed prescriber has issued you a prescription, understanding the correct injection technique is essential for safety and effectiveness. This guide walks you through the complete process of using your Mounjaro KwikPen, from storage to injection day, with practical UK-specific advice to help you feel confident administering your weekly dose.
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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 Technique
Before injecting: Remove your KwikPen from the fridge 15 minutes before use and let it reach room temperature. Check the insulin-like liquid inside — it should be clear and colourless. Never use if frozen or discoloured. Wash your hands thoroughly with soap and water.
Choosing your injection site: Mounjaro can be injected into your stomach (abdomen), upper thigh, or upper arm. Rotate your site each week to prevent lipohypertrophy (thickened skin). Avoid injecting within 2cm of your belly button or previous injection sites.
The injection: Pinch the skin gently, insert the needle at a 90-degree angle, and press the injection button slowly until you hear a click. Keep the pen in place for 3 seconds after the click, then withdraw the needle. You may see a small amount of blood — this is normal.
After injection: Place the used pen in a sharps container. Do not recap the needle. Mark your calendar for your next weekly injection day.
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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.
Storage, Doses, And What To Expect
Unopened pens: Store in a refrigerator at 2–8°C in the original carton. Do not freeze. Each KwikPen contains 4 weekly doses. Unopened pens can be used until the expiry date.
After first use: Your opened pen can be stored at room temperature (below 30°C) for up to 30 days. Dispose of it after 30 days, even if insulin remains. Never store at room temperature before opening.
Dose progression: Mounjaro starts at 2.5mg and increases gradually every 4 weeks — typically moving to 5mg, then 7.5mg, 10mg, 12.5mg, and up to the maximum 15mg. Your prescriber will guide the timing. Common side effects during the first few weeks include nausea, constipation, and reduced appetite — these usually improve over time. Track your progress using our free GLP-1 weight loss calculator to understand potential outcomes.
- 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 should I do if I miss a Mounjaro injection?
If fewer than 4 days have passed, inject as soon as you remember and continue your normal schedule. If more than 4 days have passed, skip that dose and inject on your next scheduled day. Never inject twice in one week. Always maintain at least 72 hours between injections.
Can I inject Mounjaro myself, or do I need help?
You can self-inject into your stomach or thigh. However, injecting into your upper arm requires someone else to administer it. Most people find stomach or thigh injections easier to manage independently.
How long does a Mounjaro injection take?
The actual injection takes only 5–10 seconds. However, allow 20 minutes total — 15 minutes for the pen to warm to room temperature, plus injection time itself.
What if I see blood after injecting?
Small amounts of blood are completely normal and nothing to worry about. You may have hit a small blood vessel. Apply gentle pressure with a clean tissue for a few seconds. There's no risk of reduced effectiveness.
Is a weekly weight loss injection like Mounjaro suitable for everyone?
Mounjaro is prescription-only and requires a clinical assessment. Your GP or prescriber will check your medical history, current medications, and health conditions to confirm suitability. Those with a personal or family history of thyroid cancer or multiple endocrine neoplasia should not take it.
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.