How long does a GLP-1 pen last once you start it?

Expiry dates and in-use windows, product by product

Manufacturer figures. Storage conditions and any local variation are on your leaflet, and your pharmacist is the right person to confirm them for the product you were dispensed.

One clarification first, because the two get mixed up constantly. The expiry date printed on the carton is how long the unopened pen is good for while it sits correctly stored in the fridge. The in-use window is a separate clock that starts at the first injection and runs whatever the printed date says. A pen can be comfortably inside its expiry date and still be finished as far as the in-use window is concerned, and that is the case that catches people out.

The fifth column is the one worth staring at. Every weekly four-dose pen holds the same 28 days of medicine, and the slack around it ranges from four weeks to two days.

Two days of slack is not much of a margin

On a tirzepatide KwikPen, four weekly doses is 28 days and the window is 30. That works perfectly as long as nothing moves. In practice things move constantly:

• A shot shifted by a day or two, twice, and the fourth dose is now due on day 31.
• A dose held for a week on advice, which pushes everything past the wall.
• A pen opened a couple of days before you actually needed it.
• A week where the shot slipped because of travel, illness or a supply gap.

In each of those, the pen still has medicine in it on the day it must be discarded. That is the failure mode, and almost everybody who has been on these drugs for a year has hit it at least once – usually without realising until they were counting boxes at the end of a month.

Where it really bites: a stretched dose

The collision is much bigger when someone is running a pen below its labelled dose. The arithmetic is simple and the answer is often absurd.

An Ozempic 1 mg pen holds 4 mg. Injecting 0.25 mg a week, that is sixteen weekly doses – 112 days of medicine in a pen that may be used for 56. Half the pen is unusable before you get to it, no matter how carefully you plan.

A Saxenda pen holds 18 mg. At the 3.0 mg maintenance dose that is six days, comfortably inside the 30-day window. At 0.6 mg it is thirty days, which lands exactly on the wall.

This is the calculation that almost no tracker does, and it is the reason Penwise does it automatically: it knows the labelled doses, the concentration and the in-use window for each pen, works out how many days of supply you have at the dose you are actually on, and tells you when that number is larger than the window allows. Not afterwards – before you start the pen.

The KwikPen that changed

One more thing specific to tirzepatide. The original KwikPen finishes with a measurable amount of liquid left in it after its four labelled doses, which is where the long-running “extra dose at the end” discussion comes from. Lilly began shipping a redesigned device in April 2026 that is built to finish essentially empty, so that leftover is not there any more.

Both generations are in circulation, and the packaging does not make it obvious. Penwise asks which one you have rather than assuming, because a supply count built on the wrong assumption is wrong in the direction that leaves you short.

Using leftover volume as a dose is off-label and manufacturers advise against it. Penwise knows the difference between the devices because otherwise the supply arithmetic is simply incorrect, not because it is recommending anything.

Practical habits that stop the clocks colliding

1. 01 Log the date you first used a pen, not just the doses. The window starts at first use, and nobody remembers that date three weeks later.
2. 02 Keep your injection day fixed where you can. Most collisions come from drift rather than from one dramatic disruption.
3. 03 Before opening a new pen, check which clock will run out first at your current dose. On a 30-day window that takes ten seconds and saves a box.
4. 04 Do not open a pen early. The window starts when you use it, so an unnecessary early start is days you have thrown away.
5. 05 When you hold or skip a dose, record it. A skipped week is the most common cause of a pen expiring part-full, and it is invisible in a log that only records what happened.

What Penwise tracks

For every pen you have in use, the app holds the first-use date, the doses remaining at your current strength, the days of supply that represents, and the discard date counted down. When the supply outlasts the window it says so, and when a pen is approaching its date it says that too.

It knows seventeen branded pens and five compounded vial presets, including the awkward parts: which pens have a usable leftover and which redesigned ones no longer do, how many clicks a full dose is per strength, and how long each product stays good once started. That detail is the whole reason the supply numbers can be trusted. The pen click chart covers the dosing side of the same data.

Common questions

Sources

Every factual claim on this page traces to the product information a regulator publishes, or to a published trial. Links open on the site of whoever issued the document.

1. Ozempic: EPAR product information European Medicines Agency
2. Wegovy: EPAR product information European Medicines Agency
3. Zepbound (tirzepatide) prescribing information DailyMed, U.S. National Library of Medicine

Penwise is a tracking and education tool, not a medical device. Nothing on this page is medical advice, a diagnosis or a recommendation to start, stop or change a dose. Dosing decisions belong to you and your clinician. Ozempic, Wegovy, Rybelsus, Saxenda and Victoza are trademarks of Novo Nordisk; Mounjaro, Zepbound and Trulicity are trademarks of Eli Lilly. Penwise is not affiliated with, endorsed by or connected to either company.