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
Can You Put Mounjaro Back In The Fridge After Taking It Out?
Once you remove your Mounjaro KwikPen from the fridge, you cannot put it back into cold storage. After removal, your pen must be kept at room temperature below 30°C for up to 30 days, then discarded. This is an important safety rule to follow with tirzepatide, the active ingredient in Mounjaro. Understanding proper storage is essential for keeping your prescription weight management medication safe and effective. If you're unsure about storage rules, your prescriber or pharmacist can provide clear guidance specific to your situation.
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Calculate Your BMI Instantly
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 After First Use
Once you've used your Mounjaro KwikPen for the first time, the storage rules change permanently. You cannot return the pen to the fridge at any point after removal. If you remove your pen from the refrigerator intending to use it, you must keep it at room temperature (below 30°C) for the remaining 30 days of use.
Before first use: Store unopened pens in a refrigerator between 2°C and 8°C in the original carton. After first use: Keep the pen at room temperature below 30°C for up to 30 days maximum, then dispose of it. If your pen has been left out of the fridge, do not attempt to refrigerate it again—this can damage the medication and make it unsafe.
- Unopened: Fridge (2–8°C) until expiry date
- After first injection: Room temperature only (below 30°C)
- Once removed: Cannot be refrigerated again
- Maximum 30 days after first use at room temperature
- If frozen by accident: Do not use
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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 You've Already Refrigerated It Again
If you've already put your Mounjaro pen back into the fridge after removing it, do not use that pen. Repeated temperature changes can affect the tirzepatide medication inside and compromise its safety and effectiveness. Contact your prescriber or pharmacist to report this and ask about a replacement pen.
To avoid this situation in future, consider marking your calendar on the day you first use your pen, so you know the 30-day countdown has started. Store your pen somewhere at room temperature where you'll remember it's actively in use—a bedroom drawer or kitchen cupboard works well. Never store it in direct sunlight or near heat sources like radiators or ovens. If you have questions about storage, your GP or the pharmacy that dispensed your Mounjaro can clarify the exact rules.
- 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 Mounjaro be stored at room temperature before first use?
No. Unopened Mounjaro KwikPens must be stored in a refrigerator at 2–8°C before first use. Only after you've taken the first injection can you store it at room temperature for up to 30 days.
What temperature should my Mounjaro be at once I remove it from the fridge?
Below 30°C. Room temperature storage is fine—your home or workplace temperature is typically suitable. Just keep it away from heat sources, direct sunlight, and never above 30°C.
How long can I keep Mounjaro out of the fridge after the first injection?
Up to 30 days at room temperature. After 30 days, dispose of the pen safely in a sharps bin, even if it appears to have medication left inside.
What happens if I accidentally freeze my Mounjaro pen?
Do not use a frozen pen. Freezing damages tirzepatide. Discard it and contact your prescriber or pharmacist for a replacement.
If I haven't used my Mounjaro yet, can I keep it in the fridge the whole time?
Yes. Unopened pens can stay refrigerated at 2–8°C until the expiry date printed on the carton. Once you use it for the first time, the 30-day room temperature countdown begins.
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.