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 Can Mounjaro Stay Out Of The Fridge?
Mounjaro (tirzepatide) is a prescription-only weight management injection that requires proper storage to remain safe and effective. Once you've opened a Mounjaro KwikPen, it can be safely kept at room temperature below 30°C for up to 30 days. However, unopened pens must stay refrigerated at 2°C to 8°C to maintain their stability. Understanding these storage guidelines is essential for protecting your medication and ensuring it works as intended when you inject it.
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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 Storage Rules: Room Temperature Vs Refrigerated
Unopened Mounjaro pens: Store in a refrigerator between 2°C and 8°C. Keep pens in their original carton to protect them from light. Do not freeze — if a pen has been frozen, discard it and do not use it. Unopened pens can be used until their printed expiry date if stored correctly in the fridge.
After first injection (opened pen): Once you've used your first dose from a Mounjaro KwikPen, you can store the pen at room temperature below 30°C for up to 30 days. This gives you flexibility to keep your pen accessible during your four-week dosing cycle. After 30 days, you must discard the pen, even if it contains unused doses.
Key rule: If you cannot remember whether your pen was opened more than 30 days ago, dispose of it safely. Using medication past its safe storage window may reduce effectiveness or pose a risk.
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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.
What To Do If Mounjaro Has Been Left Out Too Long
If your Mounjaro pen has been left out of the fridge at room temperature for more than 30 days after opening, it should be discarded safely — do not use it. If it's been left in a warm environment (above 30°C) for any significant time, also dispose of it. Never use a pen that has been frozen or exposed to extreme heat.
If you're unsure about your pen's storage history, contact your pharmacist or prescriber for guidance. Most UK pharmacies will replace a spoiled pen if you explain what happened, especially if it's within a reasonable timeframe of your prescription.
To avoid this situation, write the date you first open each pen on the carton and set a phone reminder for day 28 to help you track the 30-day window.
- 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 keep my Mounjaro pen in a cool bag while travelling?
Yes, but only if it stays below 30°C. Use an insulated cool bag with ice packs (not in direct contact with the pen) for short journeys. For longer travel or flights, keep your unopened pens in a purpose-made insulin travel case with cooling elements. Always keep opened pens below 30°C, and check the temperature regularly.
What if I accidentally left my Mounjaro pen out overnight?
If your opened pen was left out for just one night at normal room temperature (under 25°C), it should still be safe to use as long as you're within the 30-day window after first use. However, if it was left in a hot environment or for multiple nights, contact your pharmacist for advice.
Can Mounjaro be stored in the fridge door?
No, avoid storing Mounjaro in the fridge door. Temperature fluctuates there when the door opens and closes. Store unopened pens in the main body of the fridge at a consistent 2°C to 8°C, away from the back wall where it may be too cold.
How do I know if my Mounjaro has been damaged by temperature?
Check for visible signs: cloudiness or discolouration in the liquid (it should be clear), visible particles, or damage to the pen itself. If you notice any of these, discard it. If unsure, contact your GP or pharmacist rather than risk using compromised medication.
Do I need to bring my Mounjaro to room temperature before injecting?
Not essential, but some people find injecting easier if the pen has been at room temperature for 15–30 minutes first. This may reduce discomfort slightly, but it's not medically necessary. Never intentionally heat a pen or use warm water to warm it up.
Ready to take control of your weight loss journey?
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