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
How to Use Your Mounjaro KwikPen — Step-by-Step Instructions
If you've been prescribed Mounjaro (tirzepatide) in the UK, your medication comes in a pre-filled KwikPen device. This guide walks you through exactly how to prepare, inject, and store your pen safely. Mounjaro is a once-weekly prescription injection that requires careful handling to ensure effectiveness. Following the correct steps helps you get the most from your treatment and reduces the risk of injection problems. Always read the package leaflet that comes with your pen, and ask your GP or pharmacist if you're unsure about any step.
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Calculate Your BMI Instantly
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 Injection Instructions
Before injecting, remove your Mounjaro KwikPen from the fridge and let it sit at room temperature (below 30°C) for 15–30 minutes. Cold injections are uncomfortable. Check the medication inside the pen — it should be clear and colourless. If it looks cloudy or discoloured, do not use it.
- Step 1: Remove the pen cap
- Step 2: Wipe the injection site with an alcohol swab and let it dry
- Step 3: Hold the pen firmly and insert the needle at a 90-degree angle into your stomach, thigh, or upper arm
- Step 4: Press the button on the pen slowly until you hear a click
- Step 5: Keep the needle in your skin for 6 seconds after the click
- Step 6: Remove the needle and replace the cap
Rotate your injection site each week to avoid bruising or irritation at the same spot. Each KwikPen contains 4 doses (4 weeks' supply).
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.
Storage and Handling
Unopened pens: Store in a refrigerator at 2–8°C in the original carton. Do not freeze — if your pen has been frozen, discard it and get a replacement from your pharmacy. Keep pens away from direct sunlight.
After first use: Once you've used your pen for the first time, you can keep it at room temperature (below 30°C) for up to 30 days. After 30 days, discard the pen even if there's medication remaining. Write the date you first use each pen on the label so you know when to stop using it. Dispose of used pens in a sharps bin or return them to your pharmacy — do not put them in household rubbish.
- 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.
What should I do if I miss an injection?
If you miss your weekly Mounjaro injection, inject as soon as you remember if it's within 4 days of your usual day. If more than 4 days have passed, skip that dose and inject on your next scheduled day. Never take a double dose to make up for a missed injection. Maintain at least 3 days between injections.
Which injection sites are best for Mounjaro?
You can inject Mounjaro in your stomach (abdomen), upper thigh, or upper arm. The stomach is most commonly used. Rotate between different areas each week to prevent bruising, hardening of the skin, or lipohypertrophy (fatty tissue thickening). Never inject into an area that is tender, bruised, or scarred.
Can I share my Mounjaro pen with another person?
No, absolutely not. Mounjaro pens are single-patient use only. Sharing injection devices carries a risk of transmitting blood-borne infections. Each person must have their own prescription and their own pen. Sharing is unsafe and illegal.
What if my Mounjaro pen is damaged or leaking?
If your KwikPen is cracked, leaking, or visibly damaged, do not use it. Contact your pharmacy or prescriber immediately for a replacement. Store the damaged pen safely away from children and pets. Never attempt to repair or refill a damaged pen.
How do I know my Mounjaro injection worked?
You won't feel anything immediately after an injection — there's no obvious "sign" it worked. Effectiveness is measured over weeks and months through weight monitoring and how you feel (reduced appetite, feeling fuller after meals). Use our free BMI calculator to track your weight changes alongside your treatment, and keep a diary of any appetite changes or side effects to discuss with your GP.
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.