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
Free tracker included with every download this week
What Is a KwikPen? UK Guide to Weight Loss Injection Devices
A KwikPen is the pre-filled injection device used to deliver Mounjaro (tirzepatide) — a prescription-only weight management medicine available in the UK. It's a simple, portable pen device designed for once-weekly self-injection at home, following a clinical assessment by your GP or prescriber. Understanding how to use, store, and handle your KwikPen correctly is essential for getting the most from your treatment. This UK health guide covers everything you need to know about KwikPen devices, NHS guidance, and how they fit into a weight management plan.
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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.
How the KwikPen Works and What to Expect
The KwikPen is a pre-filled, single-use injection device supplied by Eli Lilly for Mounjaro administration. Each pen contains enough medication for one weekly dose and is colour-coded to show your dose strength. The device is designed for ease of use — you simply hold it against your skin at a 90-degree angle, press the button, and hold it in place for several seconds while the full dose is delivered.
- Each KwikPen contains exactly one dose of Mounjaro
- Colour-coded by dose: 2.5mg (grey), 5mg (blue), 7.5mg (red), 10mg (purple), 12.5mg (turquoise), 15mg (green)
- Injections take approximately 5 seconds to deliver
- Needles are small (29 gauge) to minimise discomfort
- Safe disposal boxes are provided with your supply
Your GP or prescriber will provide training on using your KwikPen before your first injection. The NHS emphasises that correct technique is important for consistent medication delivery and comfort. Always wash your hands before injecting, rotate injection sites weekly (stomach, upper thigh, or upper arm), and never reuse a pen.
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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, Safety, and UK Health Resources
Proper storage is essential for KwikPen effectiveness. Unopened pens should be kept in a refrigerator at 2–8°C and protected from light in their original carton. Once you've used a pen for the first time, it can be stored at room temperature (below 30°C) for up to 30 days. Never freeze your pens — if frozen, they cannot be used. Always check the expiry date on your pen before use.
The NHS advises reporting any injection site reactions, unusual pain, or concerns to your GP immediately. Use our free BMI calculator above to track your baseline weight and health context before starting treatment. Your GP can also refer you to NHS-approved weight management services for additional dietary and lifestyle support alongside your KwikPen injections.
- 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 do I know which KwikPen dose I should be using?
Your prescriber will tell you exactly which dose to use. KwikPens are colour-coded for easy identification, and each pen is clearly labelled with the dose strength (2.5mg to 15mg). Never change your dose without medical guidance. Your prescriber will review your progress and adjust your dose gradually over weeks.
Can I use a KwikPen more than once?
No. Each KwikPen is a single-use device containing one weekly dose. Once you've injected, dispose of the pen safely in the sharps bin provided. Never attempt to reuse a pen — this risks infection and incorrect dosing.
What if I miss my weekly KwikPen injection?
If you remember within 4 days of your missed dose, inject as soon as possible and continue your normal weekly schedule from there. If more than 4 days have passed, skip that dose and inject on your regular scheduled day. Never take a double dose. Contact your prescriber for guidance if unsure.
Where can I get KwikPen injections in the UK?
KwikPens are available through NHS specialist weight management services following a GP referral, or through private GPhC-registered online pharmacies following a clinical assessment. NHS access varies by region and may involve waiting lists. Private prescribing offers faster access without referral.
Are there any side effects from the KwikPen injection itself?
Most injection site reactions are mild — minor redness, bruising, or itching may occur. These typically resolve within hours. Serious allergic reactions are rare but require immediate medical attention. Report any unusual reactions to your GP. The KwikPen device itself is well-tolerated by most users.
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.