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 KwikPen safely and correctly
If you've been prescribed Mounjaro (tirzepatide) by your GP or through an NHS specialist weight management service, understanding the correct injection technique is essential for safety and effectiveness. The Mounjaro KwikPen is a pre-filled, single-dose auto-injection device designed for once-weekly use. This guide walks you through the process step-by-step, covering preparation, injection sites, and aftercare. Always refer to the patient information leaflet and speak to your healthcare provider if you have any concerns about your injection technique.
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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 injection technique for the KwikPen device
Before you inject: Remove your Mounjaro KwikPen from the refrigerator and allow it to warm to room temperature (15–30°C) for 15–30 minutes — do not heat it artificially. Check that the medication inside is clear and colourless. If it appears cloudy or discoloured, do not use it.
Prepare your injection site: Choose from your stomach (abdomen), upper thigh, or upper arm. Rotate injection sites weekly to prevent lipohypertrophy (thickened skin). Clean the area with an alcohol wipe and let it dry completely before injecting.
Administer the injection: Remove the blue safety guard from the needle. Pinch the skin at your chosen site, place the needle against your skin at a 90-degree angle, and press the grey button firmly until you hear a click. Keep the pen pressed against your skin for 6 seconds to ensure the full dose delivers. The needle will retract automatically once released. Replace the blue safety guard by sliding it back over the needle.
After injection: Dispose of your used KwikPen in an approved sharps bin. Do not reuse the pen — each KwikPen contains one dose only.
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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 when to contact your GP
Storage guidance: Unopened KwikPens must be refrigerated at 2–8°C and kept in their original carton. Once you start using a pen, it can be stored at room temperature (below 30°C) for up to 30 days. Discard the pen after 30 days, even if some medication remains. Never freeze Mounjaro — if it has been frozen, discard it safely.
Missed dose: If you miss your weekly injection, take it within 4 days of the scheduled date. If more than 4 days have passed, skip that dose and inject on your normal weekly day. Never take two doses in quick succession — allow at least 72 hours between doses.
Contact your GP if: You experience severe allergic reactions, persistent stomach pain, changes in vision, or signs of infection at the injection site. Your prescriber should monitor your progress and adjust your dose as needed.
- 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.
How long should I leave Mounjaro at room temperature before injecting?
Allow your KwikPen to warm to room temperature (15–30°C) for 15–30 minutes before injecting. This makes the injection more comfortable and ensures the medication flows correctly. Do not use direct heat like microwaves or hot water.
Can I inject Mounjaro into the same spot every week?
No. Always rotate your injection site weekly to prevent lipohypertrophy (thickened skin patches). Choose from your stomach, upper thigh, or upper arm. Keep a record of where you injected to help with rotation.
What should I do if I accidentally drop my Mounjaro KwikPen?
If the pen is dropped or damaged, do not use it. Dispose of it safely in a sharps bin and use a fresh pen for your next dose. The needle may be compromised even if you cannot see damage.
How do I dispose of my used Mounjaro pens?
Place used KwikPens in an approved sharps bin or yellow medical waste bin. Do not put them in regular household waste. Ask your pharmacy where to dispose of sharps if you are unsure — many GP surgeries and pharmacies provide free sharps bins.
Can I store my Mounjaro pen in a travel bag at room temperature?
Yes, for up to 30 days after first use. Unopened pens must stay in the refrigerator. Once you start using a pen, you can keep it at room temperature below 30°C for the remainder of that 30-day period, making it convenient for travel.
Ready to take control of your weight loss journey?
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