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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Mounjaro Instructions: How to Use Tirzepatide Safely in the UK
If you've been prescribed Mounjaro (tirzepatide), knowing how to use your KwikPen correctly is essential for safety and effectiveness. This guide explains step-by-step injection instructions, storage requirements, dose schedules, what to do if you miss a dose, and when to seek medical help. Mounjaro is a prescription-only once-weekly injection used alongside a reduced-calorie diet and increased physical activity. Always follow your prescriber's guidance, and contact your GP or pharmacist if you have questions about your specific treatment 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.
Step-by-Step Mounjaro Injection Instructions
Mounjaro comes as a pre-filled KwikPen device — no mixing or measuring is needed. Each pen contains four weekly doses. Here's how to inject safely:
- Check the pen: Inspect for damage, leaks, or cloudiness. The solution should be clear and colourless. Do not use if damaged.
- Wash your hands with soap and water.
- Choose an injection site: Stomach (abdomen), upper thigh, or upper arm. Rotate sites weekly to prevent lipohypertrophy (thickened skin).
- Pinch the skin: Gently pinch the area to create a fold.
- Insert the needle: Push the needle straight in at a 90-degree angle. Hold the pen steady.
- Press the injection button: Hold down until you hear two clicks (approximately 6 seconds).
- Remove the pen: Pull straight out. You may see a small drop of fluid — this is normal.
- Do not massage the injection site.
Each injection takes approximately 10 seconds. Store your pen in a refrigerator (2–8°C) until use. After your first injection, you may keep the pen at room temperature (below 30°C) for up to 30 days.
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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.
Mounjaro Dosing Schedule and Storage in the NHS Context
Mounjaro is prescribed as a once-weekly injection on the same day each week. Your NHS prescriber or specialist weight management service will advise your starting dose and escalation schedule. Doses typically begin at 2.5 mg and increase every four weeks up to a maximum of 15 mg, depending on your tolerance and response.
Storage: Unopened pens must be refrigerated at 2–8°C and kept in the original carton. Do not freeze. After first use, store at room temperature (below 30°C) for up to 30 days, then discard. Keep away from heat and direct sunlight. If you miss a dose, inject within 4 days of the scheduled date and continue your normal weekly schedule. If more than 4 days have passed, skip that dose and inject on your next scheduled day.
- 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 pen is safe to use?
Check that the pen is undamaged with no leaks or cracks. The solution inside should be clear and colourless. If it appears cloudy, discoloured, or the pen is damaged, do not use it — contact your prescriber or pharmacist for a replacement pen immediately.
Can I inject Mounjaro into any part of my body?
No. Mounjaro can only be injected into three areas: the stomach (abdomen), upper thigh, or upper arm. Rotate your injection site each week to prevent skin thickening. Never inject into areas with bruising, swelling, or hardened skin.
What should I do if I miss a weekly Mounjaro injection?
If you remember within 4 days of your scheduled injection date, take it as soon as you remember and continue your normal weekly schedule. If more than 4 days have passed since your scheduled date, skip that dose and inject on your next normal day. Never take a double dose to make up for a missed one.
How long can I keep Mounjaro at room temperature after opening?
After your first injection, you can store the pen at room temperature (below 30°C) for up to 30 days. After 30 days, the pen must be discarded even if it contains unused doses. Keep it away from heat and direct sunlight during this period.
What side effects should make me contact my GP immediately?
Seek urgent medical attention if you experience severe stomach pain spreading to your back (possible pancreatitis), serious allergic reaction (swelling of face or throat), changes in vision, or signs of gallbladder problems (pain in upper stomach). Contact your GP for persistent nausea, vomiting, or diarrhoea lasting more than a few days.
Ready to take control of your weight loss journey?
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