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 Your Mounjaro Pen: A Complete UK Guide
Using a Mounjaro KwikPen correctly is essential for safe and effective weight management. This guide walks you through each step of preparing and injecting your dose of tirzepatide, the active ingredient in Mounjaro. Whether you're prescribed this GLP-1 receptor agonist through NHS specialist services or privately, proper injection technique ensures you get the full benefit of your treatment. We'll cover preparation, injection sites, storage, and what to do if you miss a dose.
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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 Instructions for Using Your Mounjaro Pen
Before your injection: Remove your Mounjaro KwikPen from the fridge 15–30 minutes before use and let it reach room temperature. Check the liquid inside — it should be clear and colourless. Never use if it looks cloudy, discoloured, or contains particles.
Preparing your injection: Wash your hands thoroughly with soap and water. Wipe the injection site with an alcohol swab and let it dry. Remove the pen cap by twisting it off. You should see a grey button on top — this is your dose indicator.
Giving yourself the injection: Pinch the skin at your chosen injection site gently. Hold the pen at a 90-degree angle to your skin and push firmly until you hear or feel a click. Keep the pen in place for 3 seconds after the click to ensure the full dose is delivered. Remove the pen slowly and release the pinched skin. Do not rub the injection site — light pressure is fine.
After injection: Place the used pen in a sharps bin immediately. Never reuse needles or pens.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
Where to Inject and Storage Safety
Rotate your injection sites weekly to prevent lipodystrophy (abnormal fat distribution). Approved sites include your stomach (abdomen), outer thigh, or upper arm. Avoid injecting within 2 centimetres of previous injection sites, and never inject into areas with bruises, scars, or tender skin.
Storage is critical: Keep unopened pens in the fridge at 2–8°C in their original carton, away from light. Do not freeze — if a pen freezes, discard it. After first use, your pen can stay at room temperature (below 30°C) for up to 30 days. Always dispose of your pen safely in a sharps bin. If you miss a dose, inject as soon as you remember if it's within 4 days; otherwise, skip and resume your normal schedule. Consult your GP or pharmacist with any concerns about your injection technique.
- 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 without help?
The upper arm is an approved injection site, but you may find it difficult to inject yourself at a 90-degree angle. If you use your arm, ask a family member or healthcare professional to help. Stomach and thigh injections are generally easier to self-administer.
What if I accidentally inject into an area with a bruise?
Choose a different site at least 2 cm away from any bruising, scars, or tender areas. Injecting into an already-bruised area may cause additional pain or affect medication absorption. Always select healthy, unbruised skin.
How do I know if my Mounjaro pen is working properly?
You should hear or feel a click when injecting, and the grey dose indicator button will move down slightly with each injection. If the pen doesn't click or the button doesn't move, contact your pharmacist or prescriber before using it.
Can I reuse a Mounjaro needle or pen?
No. Each pen and needle is single-use only. Reusing needles increases infection risk and reduces medication sterility. Always dispose of used pens and needles in a sharps bin. Your pharmacist can provide a free sharps bin if you don't have one.
What should I do if I inject and then realise I've already had my weekly dose?
Contact your GP, pharmacist, or NHS 111 immediately for guidance. Do not inject another dose that week. Your healthcare provider will advise on whether you need any monitoring or if your next scheduled dose should be adjusted.
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.