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 The Mounjaro KwikPen
The Mounjaro KwikPen is a pre-filled, once-weekly injection device for tirzepatide, a prescription weight management medication available in the UK. This guide walks you through safe self-administration, storage, and what to expect. Whether you're starting Mounjaro or comparing it to other weight loss injections like Wegovy, understanding the correct injection technique is essential. Your prescriber should always provide initial training, but this information helps you use your KwikPen safely and confidently at home.
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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.
Mounjaro KwikPen vs Wegovy: Device and Administration Differences
Mounjaro KwikPen is a dial-and-inject device that holds 4 weeks of doses. Wegovy uses a similar pre-filled pen but delivers semaglutide instead of tirzepatide. Both are once-weekly injections, but the mechanisms differ: Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy is a GLP-1 agonist only. In clinical trials, Mounjaro (SURMOUNT trial) showed average weight loss of approximately 15–21% of body weight, compared to Wegovy (STEP trial) at around 15% — though individual results vary significantly. Both require a UK prescription following clinical assessment of your eligibility, BMI, and health conditions. Your prescriber will advise which is more suitable for you based on your medical history and any existing medications.
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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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Step-By-Step Mounjaro KwikPen Injection Instructions
1. Prepare: Remove pen from fridge 15 minutes before use. Check the liquid — it should be clear and colourless. Do not use if cloudy or discoloured. Wash your hands.
2. Choose injection site: Stomach (abdomen), thigh, or upper arm. Rotate sites weekly.
3. Clean the skin: Wipe the injection site with an alcohol swab. Let it dry.
4. Dial your dose: Turn the dial to your prescribed dose (2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg, or 15 mg). You'll hear a click.
5. Inject: Hold the pen against your skin at 90 degrees. Press the button firmly until you hear two clicks (takes 3–5 seconds). Keep pressed for 6 seconds after the second click.
6. Remove and store: Withdraw the pen and place the cover back on. Store in the fridge.
- 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 if my Mounjaro KwikPen is working correctly?
The pen should click twice during injection and take about 3–5 seconds to administer a full dose. If you don't hear the second click or the injection feels incomplete, contact your prescriber. Never re-inject the same dose. Each pen contains 4 weeks of doses — dispose of it after 4 weeks of use, even if liquid remains.
Can I store my Mounjaro KwikPen at room temperature?
Unopened pens must stay refrigerated at 2–8°C. After first use, you can keep the pen at room temperature (below 30°C) for up to 30 days. Do not freeze or expose to direct sunlight. If a pen has been frozen, discard it — do not use.
What's the difference between Mounjaro and Wegovy injection devices?
Both are pre-filled pens delivering once-weekly injections, but Mounjaro uses a dial mechanism whilst Wegovy uses a fixed-dose pen. Mounjaro contains tirzepatide (dual GIP+GLP-1 agonist), and Wegovy contains semaglutide (GLP-1 agonist). Both require a UK prescription and clinical assessment — your GP or prescriber will determine which is appropriate for you.
What should I do if I miss an injection?
If you remember within 4 days of the missed dose, take it as soon as possible and continue your normal schedule. If more than 4 days have passed, skip that dose and take your next injection on the scheduled day. Never take a double dose. Always allow at least 72 hours (3 days) between injections.
Are there any injection site reactions I should expect?
Mild redness, bruising, or soreness at the injection site is common and usually temporary. Rotate injection sites each week to minimise irritation. If you develop severe redness, swelling, or signs of infection, contact your GP. Serious allergic reactions (swelling of face, lips, or throat) require immediate medical attention.
Ready to take control of your weight loss journey?
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