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

Updated 23 August 2026 · Written by the Penwise team

Every multi-dose GLP-1 pen runs two clocks at the same time. One counts the doses still in it. The other counts the days it may be used after the first injection, whatever is left inside. Most of the time the doses run out first and nobody notices the second clock exists. The weeks where they cross are the weeks people lose medicine.

## Expiry dates and in-use windows, product by product

| Pen | Doses in it | Cadence | Days usable once started | Doses at label cadence |
| --- | --- | --- | --- | --- |
| Ozempic (all four pens) | 4 | Weekly | 56 | 28 days of doses, 28 days of slack |
| Wegovy FlexTouch | 4 | Weekly | 42 | 28 days of doses, 14 days of slack |
| Mounjaro KwikPen | 4 | Weekly | 30 | 28 days of doses, 2 days of slack |
| Zepbound KwikPen | 4 | Weekly | 30 | 28 days of doses, 2 days of slack |
| Saxenda | 6 at 3.0 mg | Daily | 30 | 6 days of doses at maintenance |
| Victoza | 10 at 1.8 mg | Daily | 30 | 10 days of doses at maintenance |
| Wegovy single-dose pen | 1 | Weekly | used at once | no in-use period |
| Mounjaro or Zepbound single-use pen | 1 | Weekly | used at once | no in-use period |

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.

> A 56-day Ozempic window is a genuinely different situation. Four weeks of slack absorbs almost any real-life disruption, which is why semaglutide users rarely think about this at all.

## 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. **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. **Keep your injection day fixed** where you can. Most collisions come from drift rather than from one dramatic disruption.
3. **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. **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. **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](https://penwiseapp.com/en/guides/glp-1-pen-click-chart) covers the dosing side of the same data.

**Never lose half a pen to a date again** Both clocks counted, for every pen you own. Free core tracker, no account, nothing sent anywhere.

## Common questions

**How long can I use an Ozempic pen after the first injection?**

The manufacturer allows 56 days from first use, whatever is left in it. With four weekly doses that leaves four weeks of slack, which is why in-use dates rarely become a problem on semaglutide.

**How long does a Mounjaro KwikPen last once started?**

Thirty days from the first injection. Four weekly doses take 28 days, so the margin is two days – which is why shifted or held doses so often push the last dose past the discard date.

**How long is a Wegovy FlexTouch pen good for after first use?**

Forty-two days. Four weekly doses take 28, leaving a fortnight of slack. The single-dose Wegovy pen has no in-use period because it is used once.

**What happens if my pen still has medicine in it on the discard date?**

The manufacturer instruction is to discard it. Penwise flags the collision in advance so you can plan around it, and does not advise using a pen past its labelled window.

**Why does a stretched dose waste medicine?**

Because the days clock does not stretch with the dose. An Ozempic 1 mg pen used at 0.25 mg a week contains 112 days of injections inside a 56-day window, so half of it expires before you reach it.

**Does the fridge extend the in-use window?**

Storage instructions vary by product and by market and they are on your leaflet. The in-use window is a separate specification from the storage temperature, and Penwise counts the window you tell it applies rather than giving storage advice.

## 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.

- [Ozempic: EPAR product information](https://www.ema.europa.eu/en/medicines/human/EPAR/ozempic), European Medicines Agency
- [Wegovy: EPAR product information](https://www.ema.europa.eu/en/medicines/human/EPAR/wegovy), European Medicines Agency
- [Zepbound (tirzepatide) prescribing information](https://dailymed.nlm.nih.gov/dailymed/search.cfm?labeltype=all&query=ZEPBOUND), 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.

## Keep reading

- [Storing a GLP-1 pen, and travelling with one](https://penwiseapp.com/en/guides/glp-1-pen-storage-and-travel): Fridge, room temperature, hand luggage and heat. The rules that hold for every pen, the ones that vary by product, and what to do when a pen has been somewhere it should not have been.
- [The GLP-1 pen click chart, and why one number never fits two pens](https://penwiseapp.com/en/guides/glp-1-pen-click-chart): What a click is actually worth on every Ozempic, Mounjaro, Wegovy, Saxenda and Victoza pen, and the one rule that makes all of it make sense.
- [Zepbound vs Mounjaro: the same drug with two labels](https://penwiseapp.com/en/guides/zepbound-vs-mounjaro): Identical molecule, identical device, identical ladder. What actually differs is the licence on the box, and that turns out to matter for cost, supply and paperwork.
- [A Zepbound tracker for the whole tirzepatide ladder](https://penwiseapp.com/en/guides/zepbound-tracker): Tracking Zepbound (tirzepatide) for weight management: six strengths, two pen designs, the thirty-day clock, and reading weight against the dose you were on.