How Long Does Mounjaro Last Once Opened?

Injectable medicines like Mounjaro (tirzepatide) and Wegovy (semaglutide) are supplied in pre-filled pens designed for single or repeated weekly use. Once the pen seal is broken or the first injection is given, the medicine may gradually degrade if exposed to temperature fluctuations, light, or simply the passage of time — even if it is stored correctly in the meantime.

What the guidance generally says

Product information for Mounjaro, as summarised on the electronic Medicines Compendium (eMC), outlines in-use storage periods after first use. Because specific figures can be updated, the most reliable source is always the patient information leaflet inside your pack or the eMC entry for your pen's strength. As a general principle: pens typically have a shorter in-use period than their overall shelf life, and this in-use window does not reset if you put the pen back in the fridge.

Storage conditions affect the in-use window

Mounjaro pens are usually stored refrigerated (2°C to 8°C) before first use, but can commonly be kept at room temperature for a limited period — again, your leaflet will give the exact figures. Once out of the fridge and in use, the time limit is fixed regardless of whether you refrigerate it again between doses. For a closer look at keeping your pen in good condition day-to-day, see our guide on storing Mounjaro in the fridge.

Expiration dates and once-opened expiry are different things

The printed expiration date on the pen is the manufacturer's outer limit under ideal, unopened storage. Once you use the pen for the first time, the in-use limit — typically shorter — takes over. Whichever date comes first is the one that matters. For more on the lifespan of a single pen across your weekly schedule, our page on how long one Mounjaro pen lasts covers that in detail, and you can also read about Mounjaro pen lifespan more broadly.

If you are ever unsure whether a pen is still within its in-use window, contact your prescriber or pharmacist before injecting — do not use a pen you have doubts about.

One of the simplest ways to avoid uncertainty about your pen's in-use window is to log the date of your first injection. GLP Trackr's dose logging feature lets you record every injection — the date, time, and dose strength — so you always have a clear record of when you first used a pen. At a glance, you can see exactly how many days have passed since that first use, without relying on memory or a sticky note on the fridge.

Beyond the date, the app also logs injection sites and tracks rotation history, so every aspect of your weekly routine is captured in one place. If you want to bring a full record of your injection history to your next prescriber appointment, the insights and doctor report features can help you do exactly that.

For a broader understanding of how this medicine works and what to expect on your journey, our hub on how Mounjaro works is a good starting point.

Always follow the guidance in your patient information leaflet and the advice of your prescriber or pharmacist — if anything about your pen's condition or storage history is unclear, check with a healthcare professional before using it. GLP Trackr records your journey; it is not a substitute for clinical advice.

Start logging your doses from day one — download GLP Trackr on the App Store or Google Play and try it free for 7 days.