The widely used convention is 28 days from first puncture, provided the vial is stored at controlled room temperature, protected from freezing and light, and each withdrawal is made with clean technique. Unopened, a sealed vial is good until the expiration date printed on the label, typically two to three years from manufacture.
Where the 28 days comes from
The 0.9% benzyl alcohol preservative is present so that a container can be entered more than once. Antimicrobial effectiveness testing (USP <51>) shows that the preservative inhibits the growth of small numbers of organisms introduced during repeated entries. The 28-day figure is a conservative, widely adopted limit for preserved multi-use containers: it gives a fixed, easy-to-track window in which the preservative can be relied on, rather than an open-ended period that would depend on how many times and how carefully the vial was entered. Where a manufacturer’s labeling states a different in-use period, the label governs.
What shortens it
- Storage outside 20–25 °C, freezing, or direct light. See temperature excursions.
- Repeated punctures of the stopper, which increase the chance of coring and of introducing organisms or particles.
- Any visible change: cloudiness, particles or color. A vial that shows these is discarded regardless of the calendar.
How laboratories track it
Write the date of first puncture and the resulting discard date on the vial label at the moment it is opened, and record both against the lot number in the reagent log. The 28-day discard-date calculator gives the date, and the SOP template shows a log format. Choosing a vial size that will be used up within 28 days avoids waste: see how labs choose a vial size.
Related
How long does bacteriostatic water last? · Does bacteriostatic water expire? · How to store bacteriostatic water
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