How Long Does a Mounjaro Pen Last?

Mounjaro is supplied as individual pre-filled KwikPens, each containing a single weekly dose at a specific strength. Once you have used a pen for your injection, it is finished — there is no second dose to draw from it later in the week.

Shelf life: unopened pens

Unopened Mounjaro pens have a shelf life specified in the patient information leaflet that comes with your medication. Always check the leaflet and the expiry date printed on your pen, but according to the electronic Medicines Compendium (eMC) product information, available on medicines.org.uk, pens should generally be stored in a refrigerator before first use. For more detail on what affects storage before you open a pen, it is worth reading about how long an unopened Mounjaro pen stays usable.

Once you have opened a pen

After you use a pen — or if a pen has been kept outside the fridge — the time it remains suitable changes. The specifics depend on the storage conditions, so always refer to your leaflet or check with your prescriber or pharmacist. You can find more guidance on how long Mounjaro lasts once a pen has been opened or removed from the fridge.

Why this matters for your weekly schedule

Because each pen covers exactly one injection, running out earlier than expected — or having a pen expire before use — means a missed dose. Keeping track of how many pens you have, when they were dispensed, and when they expire helps you stay consistent on your weekly schedule. For a closer look at the single-pen question, see how long one Mounjaro pen lasts and what that means for managing your supply.

If you have any questions about storage, shelf life, or what to do if a pen has been left out, your prescriber or pharmacist is the right person to ask — they can advise based on your specific situation.

Knowing when you used each pen — and staying on top of your weekly schedule — is exactly the kind of detail that is easy to lose track of over time. GLP Trackr's dose logging feature lets you record every injection as you go: the date, time, and dose strength, all stored in one place so you always know where you are in your treatment.

Paired with gentle dose reminders, the app helps make sure a dose is never accidentally skipped or delayed — useful when a once-a-week schedule can easily blur into a busy month. You can also log your injection site each time, so the app can suggest your next rotation and keep a clear history of where each pen was used.

If you ever do miss a dose or have a question about a pen that has been stored differently than planned, you can log a note in the app to bring to your next conversation with your prescriber. GLP Trackr is a tracking tool, not a substitute for clinical advice — any decision about dosing or pen safety belongs with your prescriber or doctor, not an app.

To understand more about how Mounjaro works as part of your weekly routine, visit our guide on how Mounjaro works.

Ready to log your doses and stay on schedule? Download GLP Trackr on the App Store or Google Play — your first 7 days are free.