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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What To Do If Your Mounjaro Isn't In The Fridge
If you've left your Mounjaro KwikPen out of the fridge, the safety of your medication depends on how long it's been at room temperature. Mounjaro has specific storage requirements, and understanding these guidelines helps you decide whether it's safe to use. This guide explains UK storage rules and what action to take if your tirzepatide pen has been left unrefrigerated.
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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 Requirements And Temperature Rules
Unopened Mounjaro pens must be stored in a refrigerator at 2°C to 8°C and must never be frozen. If a frozen pen thaws, it cannot be used safely.
After your first injection, the same pen can be kept at room temperature (below 30°C) for up to 30 days. Once 30 days have passed since first use, the pen must be discarded, even if it appears to have remaining doses.
Store your pen away from direct heat and sunlight. The original carton protects the medication from light exposure, so keep it in the box when refrigerated. If your unopened pen has been left at room temperature for a short time, it may still be safe — but this depends on how long it was out and what temperature it reached.
- Never use a pen that has been frozen
- Discard any pen more than 30 days after first use
- Keep refrigerated until first injection date
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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.
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Everything you need to understand your weight, explore medication options, and track your progress — completely free.
How Long Can Mounjaro Stay Out Of The Fridge Safely?
Unopened Mounjaro can tolerate brief periods out of the fridge, but the safety window depends on room temperature. At room temperature (around 20–25°C), an unopened pen may remain safe for a few hours, though the UK patient information leaflet does not specify an exact timeframe for unopened pens left out accidentally.
Once you have started using a pen (first injection), you have a defined 30-day window at room temperature. If you are unsure whether your unopened pen is still safe after being left out, contact your pharmacy or GP before using it. Do not attempt to use a pen if you are uncertain about its storage history. Your prescriber or pharmacist can advise whether it is safe to continue or if a replacement is needed.
- 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.
If I left my unopened Mounjaro out overnight, is it still safe to use?
If your unopened pen was left at room temperature overnight, contact your pharmacy before using it. The safety depends on the exact temperature and duration. Your pharmacist can assess whether it's still effective. When in doubt, do not use it and request a replacement.
Can I use Mounjaro after it's been left in a warm car?
If your Mounjaro has been exposed to heat above 30°C, do not use it. High temperatures can damage the medication. Contact your pharmacy or GP for guidance on obtaining a replacement pen.
What's the difference between storage rules for unopened and opened Mounjaro?
Unopened pens must stay refrigerated at 2–8°C until first use. After your first injection, the same pen can stay at room temperature (below 30°C) for up to 30 days only. After 30 days, discard it regardless of remaining doses.
Should I throw away my Mounjaro if it's been left out of the fridge?
Not necessarily. Brief exposure to room temperature may be safe for unopened pens, but contact your pharmacy to confirm. If it's been left out for many hours or exposed to heat, play it safe and do not use it. Request a replacement from your prescriber.
What should I do if my Mounjaro has been frozen?
Do not use a Mounjaro pen that has been frozen. The medication will be damaged and unsafe. Contact your GP or pharmacy immediately to report the issue and request a replacement pen.
Ready to take control of your weight loss journey?
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