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 Be Kept At Room Temperature?
Mounjaro (tirzepatide) can be safely stored at room temperature (below 30°C) for up to 30 days after you first use a KwikPen. This is important to know if you're travelling, at work, or managing your weekly injections. After 30 days at room temperature, the pen must be disposed of safely, even if it still contains medication. Proper storage helps ensure your Mounjaro remains effective and safe to use throughout your treatment.
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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.
Room Temperature Storage Rules For Mounjaro
Once you start using a Mounjaro KwikPen, you can keep it at room temperature (below 30°C) for a maximum of 30 days. This makes it convenient for weekly injections without needing refrigeration between uses. However, there are important rules to follow: Keep the pen away from direct sunlight, heat sources, and hot environments. Never leave it in a car on a warm day or near a radiator. If you cannot use the pen within 30 days, return it to the refrigerator — but once you've removed it and left it at room temperature, you cannot refrigerate it again after those 30 days are up.
- Below 30°C is safe for up to 30 days after first use
- Protect from direct sunlight and heat
- Do not refrigerate after the 30-day room temperature period
- Dispose of the pen after 30 days, even if medication remains
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.
Refrigeration Before And After First Use
Before you use your first dose, Mounjaro pens must stay in the refrigerator at 2°C to 8°C. Keep them in their original carton to protect from light. Unopened pens can be refrigerated until the expiry date printed on the carton. Once you've used your first injection from a pen, you have a choice: use it within 30 days at room temperature, or return it to the refrigerator for the remaining time before the 30-day limit. Your GP or prescriber can provide a sharps bin for safe needle disposal, or you can return used pens to a pharmacy.
If your Mounjaro has been accidentally frozen, do not use it — frozen tirzepatide cannot be reliably administered. Check the carton for the expiry date and never use medication beyond this date. Travel with your pens in an insulated cool bag if you'll be away from home for more than a few hours in warm weather. Keep a record of when you first opened each pen so you know when the 30-day limit expires. Your GP surgery or community pharmacy can answer questions about storage if you're unsure.
- 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 keeping it at room temperature for 20 days?
No. Once Mounjaro has been at room temperature for any length of time after first use, you cannot return it to the refrigerator. You must use it or dispose of it within 30 days of taking it out of the fridge. Returning it to cold storage could affect the medication's stability.
What should I do if my Mounjaro has been left in a warm car?
If it has been exposed to temperatures above 30°C for a short time, it may still be safe — but exposure to high heat can damage the medication. If you're unsure, contact your prescriber or pharmacist. When travelling, always use an insulated cool bag or ask your pharmacy for storage advice.
Do I need to keep a record of when I open my Mounjaro pen?
Yes, it's helpful to note the date you first use each KwikPen so you know when your 30-day room temperature limit expires. You can write the date on the pen carton. This ensures you don't accidentally use a pen beyond the safe storage window.
What happens if I miss a dose — can I keep the pen longer?
The 30-day room temperature limit applies regardless of whether you've missed a dose. If you've opened the pen but haven't used the next scheduled injection within 30 days, the pen must be disposed of. Contact your prescriber or pharmacist about your missed dose rather than extending storage time.
Can I use my Mounjaro dose titration schedule to avoid waste?
Your dose titration follows a set schedule (starting at 2.5 mg, increasing every 4 weeks). This is determined by your prescriber based on safety and effectiveness, not storage needs. Each pen contains four weekly doses. Storage rules don't change based on your titration timeline — follow both your prescriber's dose schedule and storage guidelines independently.
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.