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 Mounjaro KwikPen in the UK
Mounjaro (tirzepatide) comes as a pre-filled KwikPen device for once-weekly self-injection. This guide explains how to use the pen safely, store it correctly, and what to expect at each dose level. Whether you're accessing Mounjaro through NHS specialist services or a private prescription, the injection technique and storage requirements are the same. Understanding proper use ensures you get the most from your treatment.
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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 Injection Steps and Technique
Before you inject: Remove the KwikPen from the fridge 15 minutes before use and let it reach room temperature. Check the insulin-like liquid inside — it should be clear and colourless. If it looks cloudy or discoloured, do not use it.
- Clean your chosen injection site (stomach, upper thigh, or upper arm) with an alcohol wipe
- Remove the outer pen cap and grey base
- Attach the needle firmly to the pen end
- Remove both the outer and inner needle caps
- Prime the pen by dialling to 0.5 mg and pressing the injection button until a drop appears at the needle tip
- Dial to your prescribed dose (2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg, or 15 mg)
- Pinch the skin at your injection site and insert the needle at 90 degrees
- Press and hold the injection button for 3 seconds until you hear two clicks
- Remove the needle slowly and apply gentle pressure with a cotton ball if needed
Always rotate your injection site each week to prevent lipodystrophy (skin thickening or dimpling).
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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, Doses, and UK Prescribing Access
Storage: Unopened KwikPens must be refrigerated at 2–8°C and kept in their original carton away from light. Do not freeze — if frozen, the pen cannot be used. After your first injection, you can store the pen at room temperature (below 30°C) for up to 30 days. Discard your pen 30 days after first use, even if it contains remaining medication.
Dose progression: Mounjaro uses a gradual escalation schedule. You start at 2.5 mg and increase every 4 weeks if tolerated: 2.5 mg (weeks 1–4), 5 mg (weeks 5–8), 7.5 mg (weeks 9–12), and onwards to your target dose, typically 10–15 mg. Try our GLP-1 weight loss calculator to understand potential outcomes at different dose levels. In the UK, you can access Mounjaro through NHS tier 3 specialist weight management services or private prescribing after clinical assessment.
Missed doses: If you forget your weekly injection, take it within 4 days of the scheduled date and continue your normal weekly routine. If more than 4 days have passed, skip that dose and inject on your next scheduled day — never take a double dose. Always leave at least 3 days between injections.
Side effects during use: Nausea, diarrhoea, and constipation are common, especially during dose escalation. These often improve over time. Reduce food portion sizes and stick to bland foods if nausea occurs. Drink plenty of water and increase fibre intake to manage bowel changes. Contact your GP or the service prescribing your Mounjaro if side effects are severe or persistent.
- 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 inject Mounjaro myself at home?
Yes, Mounjaro is designed for self-injection at home once you've been trained by your healthcare provider. Your GP, pharmacist, or specialist nurse will demonstrate the technique before you start. You can inject into your stomach, upper thigh, or upper arm (arm injections may be easier with another person's help). Always follow the seven-step injection procedure outlined above.
What if I accidentally inject into a blood vessel?
Accidental intravascular injection is very unlikely if you follow the correct technique — inserting at 90 degrees into the pinched skin of the subcutaneous layer. If you accidentally inject into a vein, you may see blood come back through the needle. If this happens, remove the needle, apply pressure, and contact your GP or 111 for guidance. Do not re-inject the same dose on the same day.
How long does one Mounjaro KwikPen last?
Each KwikPen contains 4 weeks' worth of doses (one injection per week). So one pen lasts exactly 4 weeks. Each pen contains 1.5 mL of solution with a total of 2.4 mg of tirzepatide. You'll receive a new pen each month from your prescriber or pharmacy.
Can I use the same injection site every week?
No — rotating injection sites is important. Use a different area each week (for example: stomach week 1, left thigh week 2, right thigh week 3, upper arm week 4, then rotate back). Rotating prevents lipodystrophy (localised skin thickening or dips) and ensures even medication absorption. Keep injection sites at least 2 centimetres apart.
What's the difference between accessing Mounjaro on the NHS versus privately in the UK?
NHS access is through tier 3 specialist weight management services — you need a GP referral and may face waiting lists. NICE has approved tirzepatide for NHS use, but regional availability varies significantly. Private access is through a licensed prescriber following an online clinical assessment; it's faster but requires payment (typically £150–£250 per month depending on dose). Both routes use identical medication and injection technique.
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.