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 KwikPen Safely
If you've been prescribed Mounjaro (tirzepatide) in the UK, understanding how to use your KwikPen correctly is essential for safe and effective treatment. This guide walks you through the injection technique, storage requirements, dose timing, and what to do if you miss a dose — all within the NHS and UK prescribing context. Mounjaro is a once-weekly injectable medication, and proper use maximises both safety and effectiveness alongside a reduced-calorie diet and increased physical activity.
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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 Guide To Injecting Mounjaro
Before your first injection: Remove your KwikPen from the refrigerator 15–30 minutes before use to bring it to room temperature. Check the label to confirm you have the correct dose. Wash your hands with soap and water.
Preparing the pen: Remove the grey needle cover by pulling it straight off. Wipe the rubber stopper on top of the pen with an alcohol wipe if available. You do not need to prime the pen before use.
Choosing your injection site: Mounjaro can be injected into your stomach (abdomen), upper thigh, or upper arm. Rotate injection sites each week to prevent lipohypertrophy (thickening of fatty tissue). Avoid injecting into areas of skin that are bruised, tender, or hardened.
Injecting: Pinch a fold of skin at your chosen site. Insert the needle at a 90-degree angle (straight in). Press the injection button until it clicks and hold for 3 seconds. Remove the pen and release the skin. You may see a small amount of blood — this is normal.
After injection: Place the used pen in a sharps bin. Do not reuse needles. If you use the same KwikPen for your next dose, apply a new needle each time.
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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, Missed Doses, and NHS Prescribing Context
Storage guidance: Unopened KwikPens should be stored in a refrigerator at 2–8°C in the original carton. Once you start using a pen, you can keep it at room temperature (below 30°C) for up to 30 days. Do not freeze — if frozen, discard the pen. Each KwikPen contains four weekly doses.
Missed dose: If you remember within 4 days of a missed dose, inject as soon as you remember and continue your normal schedule the following week. If more than 4 days have passed, skip that dose and inject on your regularly scheduled day. Ensure at least 72 hours (3 days) between injections.
NHS prescribing: In the UK, Mounjaro is typically prescribed through NHS tier 3 specialist weight management services or privately through a GPhC-registered online pharmacy following clinical assessment. Your prescriber will advise on dose escalation — doses increase every 4 weeks starting from 2.5 mg up to a maximum of 15 mg weekly. Always follow your prescriber's dosing schedule exactly.
- 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 the same spot every week?
No. Rotate injection sites each week to prevent lipohypertrophy (thickening of tissue under the skin). You can use the same area of your body (e.g., stomach) but choose a different spot within that area each time. This reduces irritation and ensures consistent absorption.
What should I do if my KwikPen has been frozen?
If your Mounjaro KwikPen has been frozen, do not use it. Freezing damages the medication and makes it unsafe to inject. Contact your prescriber or pharmacy for a replacement pen. Always store unopened pens in a refrigerator at 2–8°C, never in the freezer.
How long can I keep my Mounjaro pen at room temperature after opening?
Once you've used a KwikPen for your first injection, you can store it at room temperature (below 30°C) for up to 30 days. After 30 days, discard the pen even if medication remains. Keep it in the original carton away from direct sunlight and heat.
What's the difference between Mounjaro and Wegovy in terms of UK prescribing?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections licensed in the UK. Mounjaro is a dual GIP and GLP-1 agonist given once weekly, whilst Wegovy is a GLP-1 agonist also given once weekly. Both require clinical assessment before prescription and both show similar weight loss results in clinical trials — around 15–20% of body weight on average.
Can I switch from Wegovy to Mounjaro on the NHS?
Switching between medications must be managed by your prescriber. If you're prescribed through an NHS tier 3 specialist service, your clinician will advise on whether switching is appropriate for you. There is no official NHS conversion chart between doses, so any change requires direct clinical guidance rather than self-switching.
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.