How Long Does an Opened Mounjaro Pen Last in the Fridge?

According to the electronic Medicines Compendium (eMC) product information for Mounjaro, an opened (in-use) pen should be stored in a refrigerator (2°C to 8°C) and used within the period stated in the leaflet — typically a matter of days rather than weeks. Always treat the figure in your own leaflet as definitive, since it reflects the approved product information for your specific pen.

Key storage principles for an opened pen

• Keep it in the fridge: Store between 2°C and 8°C. Do not place it directly against the freezer compartment or against the back wall, where temperatures can dip below freezing.
• Avoid light exposure: Keep the pen in its original packaging or cap to protect it from light when not in use.
• Do not freeze: A pen that has been frozen — even accidentally — should not be used. If you are unsure, contact your prescriber or a pharmacist.
• Check the in-use expiry: The leaflet will state how many days an opened pen remains usable. Note the date of first use so you can track this easily.

How this differs from an unopened pen

An unopened Mounjaro pen has a longer shelf life when kept refrigerated, and can also be stored at room temperature for a limited period if needed when travelling. For a detailed look at storage before first use, see our guide to storing an unopened Mounjaro pen, and for a broader overview of pen shelf life, how long Mounjaro lasts in the fridge covers the full picture.

It is also worth understanding how long one Mounjaro pen lasts in terms of doses — since each pen is designed for a single weekly injection, timing your storage window to your injection schedule is straightforward once you know what to look for.

One of the simplest ways to stay on top of your pen's in-use storage window is to log the date and time of every injection. GLP Trackr's dose logging feature records each injection — including the date, dose strength, and time — so you always have a clear, timestamped record of when you first opened a pen. That means no guessing whether the pen sitting in your fridge was used three days ago or six.

Dose reminders in the app also help make sure your weekly injection stays on schedule, which in turn keeps your pen usage predictable and reduces the chance of a pen sitting open and forgotten beyond its recommended window.

If you have any concern about whether your pen has been stored correctly — whether it has been left out too long, exposed to extreme temperatures, or kept past its in-use date — contact your prescriber or a pharmacist before injecting. The app can help you record what happened and when, which is useful to share with your healthcare professional. For a broader understanding of how Mounjaro works and what to track on your journey, visit our guide to how Mounjaro works.

Track every dose and never lose sight of your pen's timeline. Download GLP Trackr on the App Store or Google Play and start a free 7-day trial — your injection log is waiting.