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 Does Mounjaro Last Once Opened in the Fridge?
Once you open your Mounjaro KwikPen and use the first injection, you can store the pen in a standard refrigerator for up to 30 days. After 30 days, the pen must be disposed of safely, even if there are doses remaining. This timeline is important to follow for safety and effectiveness — keeping an opened Mounjaro pen beyond 30 days increases the risk of contamination and reduces the medication's potency. Understanding proper storage helps ensure your tirzepatide treatment remains effective throughout your weight management journey.
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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.
Storage Guidelines After Opening Your Mounjaro KwikPen
After your first injection, an opened Mounjaro KwikPen can be stored in a refrigerator (2°C to 8°C) for exactly 30 days. This applies regardless of how many doses remain in the pen. You do not need to store it in the coldest part of the fridge — a standard shelf is fine, provided the temperature stays between 2°C and 8°C.
- Keep the pen in its original carton to protect it from light exposure
- Do not freeze the pen — if accidentally frozen, discard it
- Once 30 days have passed since first use, dispose of the pen in a sharps bin or designated medicines disposal container
- If you forget when you opened the pen, write the date on the carton with a permanent marker
The 30-day limit exists because the rubber seal on the KwikPen can be punctured each time you inject, potentially allowing bacteria to enter. Staying within this timeframe maintains the sterility and stability of the tirzepatide solution.
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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.
Temperature and Room Storage — What If Your Fridge Breaks?
If your refrigerator stops working or reaches temperatures above 8°C, an opened Mounjaro pen can be stored at room temperature (below 30°C) for up to 30 days from the date of first use — but only if it has not previously been refrigerated continuously. However, the safest approach is to keep it refrigerated whenever possible.
If the pen has been exposed to temperatures above 30°C or below 2°C for extended periods, discard it. Do not attempt to re-refrigerate a pen that has been left at room temperature. Always document the storage conditions and dates to avoid confusion. If you are unsure whether your pen is still safe, contact your prescriber or pharmacist — they can advise whether 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.
Can I store an opened Mounjaro pen at room temperature instead of the fridge?
Yes, but only as a temporary measure. An opened pen can be kept at room temperature below 30°C for up to 30 days from first use. However, refrigeration (2°C to 8°C) is preferred for maximum stability. Never re-refrigerate a pen once it has been at room temperature — choose one storage method and stick with it for the full 30 days.
What happens if I leave my opened Mounjaro in the fridge for more than 30 days?
The medication becomes unsafe to use after 30 days from first injection, even if doses remain. The rubber seal becomes compromised, risking contamination. Always dispose of the pen after 30 days using a sharps bin or NHS medicines disposal service — never throw it in household waste.
How do I know when my Mounjaro pen was first opened?
Write the date you first inject directly on the KwikPen carton in permanent marker. This prevents confusion and ensures you do not accidentally use the pen beyond the 30-day window. Many people also set a phone reminder for day 25 as a safety alert.
Should I use the weight loss calculator to track my Mounjaro progress?
Yes. MyWeightCheck's free GLP-1 weight loss calculator can help you estimate potential outcomes at your current dose and track progress over weeks. Combined with our BMI calculator and progress tracker, these tools give you a complete picture of your weight management journey alongside proper medication storage.
Does an unopened Mounjaro KwikPen last longer than an opened one?
Yes. Unopened pens stored in the fridge (2°C to 8°C) remain stable until the expiry date printed on the box — typically several years. Once opened and the first dose used, the 30-day clock starts, regardless of how many weeks until the printed expiry date.
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.