How Long Does Mounjaro Last After Opening?

An unopened Mounjaro pen kept under the right conditions retains its stability up to the expiry date printed on the label — for more on that, see our guide to Mounjaro shelf life and storage conditions. Once in use, however, the clock resets to a shorter in-use period, typically around 30 days, regardless of when the pen was manufactured. Always verify the exact figure in the leaflet provided with your specific pack, or on the eMC at medicines.org.uk, as guidance can be updated.

Storage conditions during the in-use period

Keeping your pen stored correctly during this period is just as important as noting the date you first used it. The general guidance for Mounjaro pens in use is to store them below 30°C, away from direct heat and light. They should not be frozen at any point — a pen that has been frozen should not be used, and you should contact your prescriber or pharmacist for advice. Replacing the cap after each use also helps protect the pen from light and contamination.

Why the in-use shelf life matters practically

Because Mounjaro is a once-weekly injection, a single pen typically delivers multiple doses over several weeks. If you start a new pen and then have a gap — perhaps due to illness, travel, or a change in routine — it is worth noting the date you first used it so you can check whether it is still within the recommended window at your next scheduled dose. If the pen has passed its in-use limit, do not use it; contact your prescriber or doctor for guidance. For context on how the medicine behaves more broadly after a dose, our page on what happens after a Mounjaro injection covers the post-dose period in more detail.

One of the most practical things GLP Trackr does is give you a clear, dated log of every injection — the date, time, and dose strength recorded in one place. When you can see exactly when you opened a pen and when each dose was taken, checking whether you are within the recommended in-use window becomes straightforward rather than a matter of guesswork.

Alongside dose logging, the app's dose reminders help keep your weekly schedule on track, reducing the risk of accidental gaps that might leave you unsure how long a pen has been in use. And if you do experience any unexpected changes — in how the pen looks, or how you feel around injection time — the side-effect and symptom log lets you record those observations to bring to your next prescriber appointment.

If you ever have doubts about a pen's condition or whether it is still within its in-use period, always follow the guidance of your prescriber, doctor, or pharmacist — they can advise based on your specific situation. The app supports your tracking; it is not a substitute for that professional guidance.

You can read more about the broader topic on our Mounjaro information hub, or explore how Wegovy's half-life compares if you are curious about how semaglutide behaves differently in the body.

Keep a clear log of every pen you open — download GLP Trackr on the App Store or Google Play and start your free 7-day trial today.