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 Set Up Your Mounjaro KwikPen
Setting up your Mounjaro pen correctly is essential for safe and effective treatment. Mounjaro (tirzepatide) is a prescription-only, once-weekly injectable medicine delivered via a pre-filled KwikPen device. This guide walks you through each step of preparing and injecting your dose, from checking your pen to selecting your injection site. Always follow the instructions provided by your prescriber and the patient information leaflet included with your medication. If you have any questions about your technique, contact your GP or the pharmacy that dispensed 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 Pen Setup Guide
Before you inject, gather everything you need: your Mounjaro KwikPen, a new needle (provided with your prescription), an alcohol wipe, a sharps bin for safe needle disposal, and a clock or timer. Check that your pen has not been frozen and the expiry date has not passed. Remove the pen from the fridge 15 minutes before use and allow it to warm to room temperature — this makes injection more comfortable. Inspect the medication through the window on the side of the pen; it should be clear and colourless. If it looks cloudy, discoloured, or contains particles, do not use it and contact your pharmacy. Screw a new needle straight onto the pen until it clicks firmly into place. Remove both the outer and inner needle covers by twisting them off. Set the dose selector to your prescribed dose (2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg, or 15 mg) — you will hear a click as you turn the dial. Check the dose window to confirm your selected dose is visible. You are now ready to inject.
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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.
Injection Technique and Aftercare
Choose your injection site: stomach (abdomen), upper thigh, or upper arm. Rotate your injection site each week to avoid irritation. Clean the skin with an alcohol wipe and allow it to dry. Pinch the skin gently, insert the needle at a 90-degree angle, and press the injection button firmly until you hear a click and see the yellow indicator appear in the dose window. Keep the needle in your skin for 3 seconds after clicking to ensure the full dose is delivered. Remove the needle carefully and apply gentle pressure with a dry cotton pad if needed. Put the used needle into a sharps bin immediately — never reuse needles. Store your pen in the fridge between 2–8°C until your next injection. If you miss a dose, inject within 4 days of the missed date; if more than 4 days have passed, skip that dose and inject on your normal scheduled day.
- 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 reuse my Mounjaro needle?
No. Always use a new sterile needle for each injection. Reusing needles increases infection risk and may damage the needle, preventing proper dose delivery. Dispose of used needles in a sharps bin and request new needles with your next prescription.
What if I inject air into my skin by mistake?
Small air bubbles are not dangerous. If you see a large air bubble in the pen before injecting, hold the pen upright and tap it gently to move the bubble to the top, then press the injection button slightly to release the air before injecting your dose.
How do I know if my Mounjaro pen is frozen and shouldn't be used?
If your pen has been frozen, the medication will appear cloudy or icy. Frozen pens should not be used. Do not attempt to thaw a frozen pen; contact your pharmacy for a replacement.
What should I do if my Mounjaro pen is damaged or leaking?
Do not use a damaged pen. Contact the pharmacy that dispensed your prescription immediately for a replacement. If the needle is bent or broken, do not attempt to use it; request a new needle when you collect your next dose.
Can I travel with my Mounjaro pen on a plane?
Yes. Keep your pen in your hand luggage in a small insulated travel case with ice packs. Inform airport security that you are carrying a prescription injectable medication. Do not pack it in checked luggage as extreme temperatures may damage 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.