How long is bacteriostatic water good for after opening?

28 days is the figure to work to, but not for the reason usually given. The manufacturer's label sets no in-use period at all. USP chapter 797, the pharmacy compounding standard, gives a beyond-use date of 28 days for a punctured multi-dose container unless the manufacturer specifies otherwise, and since the manufacturer specifies nothing, 28 days is what applies.

Is the 28 days printed on the vial?

No. Reading the full Hospira label turns up no in-use period, no discard-after date and no mention of 28 days anywhere. The only storage instruction it gives is controlled room temperature, and it does not distinguish between an unopened and a punctured vial.

Can the 28 days extend past the printed expiry date?

No. A beyond-use date can never be later than the expiry printed on the vial. If the vial expires in twelve days, puncturing it does not buy another 28; you have twelve.

How should bacteriostatic water be stored?

At controlled room temperature, 20 to 25 degrees Celsius. The label gives the same instruction before and after the stopper is first entered and does not ask for refrigeration.

Does the same 28 days apply to the reconstituted peptide?

It is a different question with a different answer, and the bacteriostatic water label is explicit about not answering it. The label says not to store reconstituted drug solutions unless the manufacturer of the drug being dissolved directs otherwise, which places the question with that product rather than with the diluent.

Bacteriostatic Water Shelf Life

28 days is the right answer. It is not on the label, and where it actually comes from changes what it means.

The label sets no in-use limit

Read the full FDA-approved package insert for Bacteriostatic Water for Injection and there is no in-use period in it. No discard-after date, no 28 days, no distinction between a sealed vial and one that has been entered forty times. The only storage instruction is controlled room temperature, 20 to 25 degrees Celsius.

This surprises people, because the figure is quoted so consistently across this niche that it reads like a manufacturer instruction. It is not one.

Source: Bacteriostatic Water for Injection, USP. Hospira, Inc. FDA-approved package insert, via DailyMed. Read 2026-09-20.

Where 28 days actually comes from

USP <797>, the United States Pharmacopeia chapter on compounding sterile preparations, sets a beyond-use date of 28 days for a multi-dose container once it has been entered, unless the manufacturer specifies otherwise.

That clause is the whole answer. The manufacturer specifies nothing, so the default stands. The 28 days is not folklore and it is not a label instruction either; it is a compounding standard filling a silence, and it applies for exactly that reason.

The reason a multi-dose vial gets 28 days and a single-dose vial gets none is the preservative. Benzyl alcohol is what makes repeated entry defensible at all, which is covered on the bacteriostatic water page.

Source: General Chapter <797> Pharmaceutical Compounding, Sterile Preparations. United States Pharmacopeia. Read 2026-09-20.

The ceiling the 28 days cannot pass

A beyond-use date is never later than the expiry date printed on the vial. Puncturing a vial that expires in twelve days does not start a fresh 28 day clock. You get the twelve.

So the working rule is the earlier of two dates: 28 days from first entry, or the printed expiry. Writing the first-entry date on the vial with a marker is the only way to know which applies later, and it is the step that gets skipped.

What the label does say about storing mixtures

On the question people actually want answered, which is how long the reconstituted vial lasts, the label is direct and unhelpful in equal measure. It says, in full:

Do not store reconstituted solutions of drugs for injection unless otherwise directed by the manufacturer of the solute.

It also asks for aseptic technique on every entry, and says that when diluting or dissolving drugs you should mix thoroughly and use promptly.

That is the diluent manufacturer declining to answer for a product it knows nothing about, which is reasonable of them and leaves the question where it belongs: with whatever is being dissolved. The storage page covers what is generally understood about reconstituted peptides, and is careful about which parts of that are documented and which are convention.

Source: Bacteriostatic Water for Injection, USP. Hospira, Inc. FDA-approved package insert, via DailyMed. Read 2026-09-20.

Questions about shelf life

How long is bacteriostatic water good for after opening?: 28 days is the figure to work to, but not for the reason usually given. The manufacturer's label sets no in-use period at all. USP chapter 797, the pharmacy compounding standard, gives a beyond-use date of 28 days for a punctured multi-dose container unless the manufacturer specifies otherwise, and since the manufacturer specifies nothing, 28 days is what applies.
Is the 28 days printed on the vial?: No. Reading the full Hospira label turns up no in-use period, no discard-after date and no mention of 28 days anywhere. The only storage instruction it gives is controlled room temperature, and it does not distinguish between an unopened and a punctured vial.
Can the 28 days extend past the printed expiry date?: No. A beyond-use date can never be later than the expiry printed on the vial. If the vial expires in twelve days, puncturing it does not buy another 28; you have twelve.
How should bacteriostatic water be stored?: At controlled room temperature, 20 to 25 degrees Celsius. The label gives the same instruction before and after the stopper is first entered and does not ask for refrigeration.
Does the same 28 days apply to the reconstituted peptide?: It is a different question with a different answer, and the bacteriostatic water label is explicit about not answering it. The label says not to store reconstituted drug solutions unless the manufacturer of the drug being dissolved directs otherwise, which places the question with that product rather than with the diluent.