Insulin Expiration Date Calculator

Why “28 days” is the wrong answer for half of these products

Almost every storage page online gives one number: discard 28 days after first use. That figure is correct for rapid-acting analogues, and it is wrong — in both directions — for a large share of everything else.

Get it wrong one way and you waste insulin. A Tresiba or Toujeo pen discarded at 28 days is thrown out with four usable weeks left in it, which at current prices is a meaningful amount of money for something that was still perfectly good.

Get it wrong the other way and you inject insulin that has started to lose potency. Premixed pens are the case that matters here. A Humalog Mix or Humulin 70/30 pen is a 10-day product; NovoLog Mix 70/30 FlexPen is 14 days. Someone applying the familiar 28-day rule to a premixed pen is using it for two extra weeks past its limit — and because degraded insulin fails silently, the only sign is blood glucose that runs high for no apparent reason.

In-use days after first use, by product

Vial and pen figures are listed separately because they genuinely differ. The final column is the temperature ceiling that in-use window assumes — note that the Novolin range uses a lower limit than the analogues.

A dash means that product is not marketed in that container. Where a manufacturer gives a range — Humulin 70/30 cartridges are quoted at 7–10 days — this table uses the longer figure and you should treat the shorter one as the safer assumption.

A worked example, and the mistake it prevents

Take someone on a basal-bolus regimen who opens two products on the same morning, 1 March: a Tresiba FlexTouch for basal and a NovoLog Mix 70/30 FlexPen they use for a fixed evening meal.

Applying the familiar 28-day rule to both gives a single discard date of 29 March. Both figures are wrong.

• The Tresiba pen has a 56-day window, so its real discard date is 26 April. Throwing it out on 29 March discards four usable weeks of a long-acting insulin.
• The NovoLog Mix FlexPen has a 14-day window, so its real discard date is 15 March. Continuing to 29 March means two extra weeks on a pen that is past its limit.

The second error is the one that matters clinically. There is no visual change to warn you — the pen looks identical on day 28 as it did on day 1. What you see instead is evening readings drifting upward for no obvious reason, which is easily mistaken for a carb-counting problem or a ratio that needs changing. The dose is fine; the insulin is simply weaker than it was.

This is why the calculator asks for the container as well as the product. For most insulins it makes no difference. For the premixes it changes the answer by two weeks.

Unopened versus in use

Two separate clocks apply, and only one of them is printed on the box.

The row people miss is the second one. Leaving an unopened pen on the counter starts the in-use clock even though you have not injected with it yet — so a spare kept out of the fridge “ready to go” may already be partway through its window.

Two different temperature ceilings

In-use windows are only valid below a stated temperature, and that temperature is not the same for every product. The analogues — Humalog, NovoLog, Lantus, Levemir, Tresiba, Toujeo — are specified below 86°F (30°C). The Novolin human insulins are specified below 77°F (25°C), which is a genuinely different instruction and one that ordinary indoor temperatures can exceed in summer.

Refrigerated storage is 36–46°F (2–8°C), and that is for unopened product. Once a pen or cartridge is in use it is meant to stay at room temperature — Novo Nordisk specifically states that in-use Novolin vials should not be returned to the refrigerator, and that in-use pens and cartridges should be kept out of it, without a needle attached.

If it was left out, overheated, or frozen

Left out beyond the window. The in-use clock does not pause. Insulin that has sat at room temperature past its discard date has lost an unknown amount of potency, and no inspection will tell you how much. Replace it.

Overheated. Sustained heat above the product ceiling degrades insulin considerably faster than the calendar assumes. Cars, windowsills, beach bags and checked luggage are the usual culprits. If insulin has been somewhere genuinely hot, treat the calculated date as void.

Frozen. Freezing destroys insulin permanently and irreversibly, and frozen-then-thawed insulin often looks completely normal. That is exactly what makes it dangerous. Discard it — including anything that was pressed against the back wall of a fridge, which is colder than the shelf.

In every one of these cases the practical risk is the same: the insulin does not look wrong, it simply works less well, and the first sign is unexplained high readings. If you are troubleshooting stubborn highs and cannot find a cause, a compromised vial is worth ruling out.

Limitations & Safety Notes

• This calculator uses published in-use windows for common products sold in the United States. Formulations, brand names and stated windows differ by country — the leaflet supplied with your product always overrides this table.
• The in-use window assumes storage below the stated temperature for the whole period. It cannot account for a hot car, a sunny windowsill or a day at the beach, all of which shorten it by an unknown amount.
• Biosimilars and newer products are not all listed here. If your product is not in the dropdown, use the figure printed in its leaflet rather than a similar-sounding brand.
• The calculated date is the point at which to discard, not a guarantee of full potency up to it. Insulin degrades gradually, so a vial in its final days may already be slightly weaker than a fresh one.
• Discarding on time is a supply and safety question, not a dosing one. If your glucose is running unexpectedly high, speak to your care team rather than adjusting doses on the assumption that the insulin is at fault.

Frequently Asked Questions