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 4 Dose Pen Safely
If you've been prescribed Mounjaro (tirzepatide) in the UK, your medication arrives as a pre-filled KwikPen containing 4 once-weekly doses. Using the pen correctly is essential for both safety and effectiveness. This guide walks you through the injection technique, storage requirements, injection site rotation, and what to do if you make a mistake. Whether you're starting your dose titration journey or moving to a higher dose, understanding your KwikPen ensures you get the most from your prescription.
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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.
Step-by-Step Guide to Using Your Mounjaro KwikPen
Before injecting, remove your KwikPen from the fridge and let it sit at room temperature for 15 minutes. Check the liquid inside — it should be clear and colourless. If it looks cloudy, discoloured, or contains particles, do not use it and contact your prescriber. Wash your hands thoroughly.
Choose your injection site: stomach (abdomen), upper thigh, or upper arm. Rotate sites each week to prevent skin irritation. Pinch the skin gently, insert the needle at a 90-degree angle, and press the grey injection button until you hear or feel a click. Hold in place for 3 seconds, then release and remove the needle. You may see a small amount of blood or clear fluid — this is normal.
Keep the pen upright during storage at 2–8°C in the fridge. Once you start using it, the pen can be kept at room temperature (below 30°C) for up to 30 days. Dispose of your pen 30 days after first use, even if medication remains.
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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.
NHS Access and What to Expect on Mounjaro
In the UK, Mounjaro is available through NHS specialist tier 3 weight management services following referral from your GP, or through private prescribing. NICE has approved tirzepatide for use within NHS services for adults with a BMI of 30 or above, or BMI 27–29.9 with weight-related health conditions like type 2 diabetes or high blood pressure.
Your prescriber will start you on 2.5 mg and increase your dose every 4 weeks if well tolerated, up to a maximum of 15 mg. Common side effects include nausea, which often improves as your body adjusts. Track your progress using our GLP-1 weight loss calculator to see projected outcomes, and use our BMI calculator to monitor changes in your BMI category as you progress through your dose titration.
- 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've missed my weekly Mounjaro injection?
If you remember within 4 days of the missed dose, inject as soon as possible and continue your normal schedule the following week. If more than 4 days have passed, skip that dose and take your next injection on your normal day. Never double-dose or inject before 3 days have passed since your last injection. Contact your prescriber if you're unsure.
Can I inject Mounjaro into my upper arm myself?
The upper arm is an approved injection site, but it's technically easier to inject with assistance from another person because the angle is difficult to manage alone. Your stomach or thigh are better choices for self-injection. If you choose to inject your arm, ensure you have full visibility and use a mirror if needed.
What are the main side effects of Mounjaro in UK patients?
The most common side effects are nausea, diarrhoea, constipation, and stomach pain. These often occur during dose titration and usually improve within a few days or weeks as your body adjusts. Serious side effects are rare but include severe stomach pain (possible pancreatitis) or serious allergic reactions — seek immediate medical attention if these occur.
How long can I keep my Mounjaro pen at room temperature?
Once you've started using your KwikPen, it can be stored at room temperature (below 30°C) for up to 30 days. This is helpful when travelling within the UK or going on holiday. Always store unopened pens in the fridge at 2–8°C in the original carton. Never freeze your pen — if frozen, it cannot be used.
Is Mounjaro the same as Wegovy?
Both are weight loss injections available in the UK, but they work differently. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, while Wegovy (semaglutide) is a GLP-1 agonist only. Mounjaro is dosed once weekly, and clinical trials suggest it may produce slightly greater average weight loss, though individual results vary significantly.
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.