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 Happens If Mounjaro Is Not Refrigerated?
If your Mounjaro KwikPen is left unrefrigerated, it may become damaged and unsafe to use, depending on how long and at what temperature it was stored. This guide explains Mounjaro storage requirements, what temperatures damage the medication, how to tell if your pen is still safe, and what to do if you've accidentally left it out. Proper storage is essential to maintain the effectiveness of tirzepatide, the active ingredient in Mounjaro.
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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 Temperature and Storage Requirements
Unopened Mounjaro pens must be stored in a refrigerator at 2°C to 8°C (standard fridge temperature). They should never be frozen — if a pen freezes, it becomes unusable and must be discarded. Keep unopened pens in their original carton to protect them from light.
- Store in the main body of your fridge, not the door (temperature fluctuates at the door)
- Keep away from the freezer compartment
- Do not expose to direct sunlight
- Check the expiry date — unopened pens can be used until the printed expiry date if kept refrigerated
After your first injection, your Mounjaro KwikPen can be kept at room temperature below 30°C for up to 30 days. Once 30 days have passed since first use, dispose of the pen even if it contains unused doses. This flexibility allows you to carry your pen with you for weekly injections without constant refrigeration.
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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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What Temperature Damage Does to Mounjaro
If your unopened Mounjaro pen is exposed to temperatures above 30°C for prolonged periods, the medication inside may degrade and become less effective or unsafe. Temperatures between 8°C and room temperature (not frozen) for short periods — a few hours to a day — typically do not cause permanent damage, but extended exposure to heat will.
If you accidentally leave an unopened pen unrefrigerated overnight or for several hours, contact your pharmacy or prescriber for guidance. Never assume the pen is still safe. If a pen has been left in a hot environment (car, direct sunlight, above 30°C for more than a few hours), it should be discarded and replaced. Do not use it for injection, as the tirzepatide formulation may be compromised, affecting both safety and effectiveness of your dose titration plan.
If you accidentally leave an unopened pen unrefrigerated overnight or for several hours, contact your pharmacy or prescriber for guidance. Never assume the pen is still safe. If a pen has been left in a hot environment (car, direct sunlight, above 30°C for more than a few hours), it should be discarded and replaced. Do not use it for injection, as the tirzepatide formulation may be compromised, affecting both safety and effectiveness of your dose titration plan.
- 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 use Mounjaro if it's been left out of the fridge overnight?
Not without checking with your pharmacist or prescriber first. Unopened pens must stay at 2–8°C. If left at room temperature overnight, the pen may be unsafe to use. Contact your pharmacy immediately — they can advise whether the pen is still usable based on the exact temperature and duration. Never guess.
What's the difference between storage before and after first use?
Unopened pens must be refrigerated at 2–8°C. After your first injection, the same pen can be kept below 30°C for up to 30 days, making it portable for weekly injections. After 30 days from first use, discard the pen regardless of remaining doses.
If my Mounjaro pen froze, can I still use it?
No. Frozen Mounjaro cannot be used and must be discarded. Freezing damages the tirzepatide formulation irreversibly. Always store pens in the main fridge body, never in the freezer compartment.
How do I know if my Mounjaro pen has been damaged by heat?
You cannot always tell by looking. If a pen has been exposed to temperatures above 30°C for extended periods, or left in direct sunlight or a hot car, assume it's damaged. Contact your pharmacy for a replacement rather than risk using compromised medication.
Can I travel with Mounjaro and keep it cool?
Yes. After first use, your pen can be kept below 30°C for up to 30 days, making travel easier. For longer trips, use a cool bag with ice packs (ensure the pen doesn't freeze). For flights, keep your pen in your hand luggage in a cool bag — check airline rules for carrying prescription medicines.
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.