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 Long Does Mounjaro Last After Opening?
Once you open a Mounjaro KwikPen and use your first injection, the pen can be stored safely for up to 30 days at room temperature. This is an important timeframe to remember if you're using tirzepatide for weight management in the UK. Understanding proper storage after opening helps ensure your medication remains safe and effective throughout your treatment journey.
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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.
Storage After Opening — The 30-Day Rule
After your first injection, an opened Mounjaro KwikPen can be stored at room temperature (below 30°C) for a maximum of 30 days. This is a critical timeframe for UK patients using tirzepatide as a prescription-only weight management injection. The 30-day period starts from when you first use the pen, not from when you receive it. If you've opened your pen but haven't injected yet, the 30-day countdown begins once you take that first dose. Keep the pen away from direct heat and sunlight during this period. After 30 days, you must dispose of the pen safely — even if there are doses remaining inside. Do not attempt to extend the timeframe or store it differently, as this could affect the medication's safety and effectiveness.
- Store opened pens below 30°C (room temperature)
- Maximum storage time: 30 days from first use
- Keep away from direct sunlight and heat
- Dispose of after 30 days, even if doses remain
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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.
Before Opening — Refrigerator Storage Guidelines
Unopened Mounjaro KwikPens must be stored in a refrigerator at 2–8°C. This is how your tirzepatide pen arrives and how it should be kept until you're ready to use it. Store unopened pens in the original carton to protect them from light exposure. Never freeze Mounjaro — if a pen is accidentally frozen, do not use it. Check the expiry date on the carton; unopened pens can be used until this date if refrigerated correctly. Once you decide to start treatment, remove a pen from the fridge and use it within 30 days at room temperature. This two-stage storage system (refrigerated before opening, room temperature after opening) is designed to keep your prescription weight management medication safe and effective throughout your treatment.
- 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 refrigerate Mounjaro after opening it?
No. Once opened, Mounjaro must be stored at room temperature below 30°C for up to 30 days. Returning it to the fridge after use may affect the medication's stability. Always follow the 30-day room temperature rule after your first injection.
What happens if I leave my opened Mounjaro pen out for longer than 30 days?
After 30 days at room temperature, the tirzepatide may become unstable and unsafe. You must dispose of the pen, even if doses remain. Using expired medication could be ineffective or potentially harmful. Always follow the 30-day limit strictly.
Do I need to keep the original carton after opening?
You don't need the carton once you've opened the pen, but keeping it can be helpful for reference (it contains your expiry date and batch information). Store the opened pen safely away from light and heat, in a cool, dry place below 30°C.
How do I dispose of my used Mounjaro KwikPen?
Return used or expired pens to a pharmacy or GP surgery in the UK — never put them in household bins. They'll dispose of it safely. Your prescriber or pharmacist can also advise on disposal when you collect your prescription.
What's the difference between unopened and opened Mounjaro storage times?
Unopened pens last until the expiry date (stored at 2–8°C in the fridge). Opened pens last only 30 days at room temperature. This difference reflects how the medication degrades once the pen is in use and exposed to air.
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.