How Long Does One Pen of Mounjaro Last?

Mounjaro (tirzepatide) is a prescription-only, once-weekly injectable medicine supplied as pre-filled pens. Each pen holds one measured dose at a specific strength — commonly available in strengths of 2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg, and 15 mg — and is engineered for a single injection. Once activated and used, the pen cannot be reloaded or reused.

Because the dosing schedule is once per week, one pen equates to one week of treatment. A standard monthly supply therefore typically means four pens, though the exact number you receive at any one time depends on how your prescription is dispensed — your prescriber or dispensing pharmacy can tell you what to expect.

Does unused solution mean there's a second dose in the pen?

Sometimes a small amount of liquid may remain visible after an injection — this is normal and does not represent a usable second dose. The pen is calibrated to deliver one complete dose per use; any residual liquid is not intended to be injected. If you have concerns about whether a full dose was delivered, speak with your prescriber or pharmacist rather than attempting a repeat injection.

How long does an unused pen stay usable?

An unopened Mounjaro pen has a shelf life set by the manufacturer — check the expiry date printed on the pen and outer packaging, and refer to the patient information leaflet or the electronic Medicines Compendium (eMC) for authoritative storage guidance. How you store the pen between receiving it and using it also affects whether it remains usable; for detail on that, see our guide to storing your Mounjaro pen correctly.

For a broader look at how long a Mounjaro pen lasts — covering both duration of use and storage life — that page brings together both questions in one place. You may also find it useful to read about making the most of each pen as part of your weekly routine.

Knowing that one pen equals one dose equals one week sounds simple — but in practice, staying on top of your weekly schedule, remembering exactly when you last injected, and spotting if a dose was missed is harder than it sounds. That's where dose logging becomes genuinely useful.

GLP Trackr lets you record every injection the moment you give it: the date, time, and dose strength are all saved in one place, giving you a clear, timestamped history of every pen you've used. If you ever wonder "did I already do this week's injection?" — your log has the answer. The app also sends gentle reminders so your once-weekly schedule stays on track and no pen goes to waste through a forgotten dose.

Beyond logging individual shots, the insights dashboard shows your injection history at a glance, which can be a useful reference to bring to your next prescriber appointment — particularly if you want to discuss how your schedule has been going or whether any doses were delayed.

To understand more about how Mounjaro works and why the once-weekly schedule is structured the way it is, visit our guide to how Mounjaro works.

A note on missed doses: if you think you may have missed an injection or are unsure whether a dose was fully delivered, the right step is to check the patient information leaflet and contact your prescriber or pharmacist — not to re-inject. GLP Trackr can help you log the situation so you have an accurate record to discuss with them, but any decision about what to do next belongs with your healthcare professional.

Ready to take the guesswork out of your weekly injection routine? Download GLP Trackr on the App Store or Google Play and explore it free for 7 days — your dose history, reminders, and progress all in one place.