Practical Guide

How Long Does Bacteriostatic Water Last?

Bacteriostatic water for injection (BAC water) is sterile water containing 0.9% benzyl alcohol as a preservative. The preservative slows microbial growth so a vial can be punctured multiple times during peptide reconstitution — but only for a limited window. Here is what the USP guidance and manufacturer labelling actually say.

Shelf life at a glance

Sealed, unopened vial

Until printed expiry (typically 18–36 months)

Stored at room temperature, away from direct sunlight.

Opened vial, refrigerated (2–8°C)

Up to 28 days

USP standard for benzyl alcohol bacteriostatic water once first puncture is made.

Opened vial, room temperature

Up to 28 days

Same 28-day window applies; refrigeration is preferred but not required.

After 28 days from first puncture

Discard

The 0.9% benzyl alcohol preservative loses reliable bacteriostatic action; risk of microbial growth rises sharply.

Why 28 days?

The 28-day rule comes from the United States Pharmacopeia (USP) guidance for multi-dose vials containing antimicrobial preservatives. Benzyl alcohol at 0.9% inhibits — but does not kill — bacteria, fungi, and yeast. With each needle puncture, microbes can be introduced; the preservative buys time, but its protective margin degrades after roughly four weeks.

Manufacturers (Hospira, Pfizer, and the major compounders) print this 28-day discard window directly on the vial label. It is the same number used for hospital pharmacy compliance and is the safest default for research peptide reconstitution.

Discard immediately if you see

• Cloudiness, particles, or visible floaters in the vial
• A change in colour (BAC water should be perfectly clear)
• A damaged or compromised rubber stopper
• Any uncertainty about how long ago the vial was first punctured

When in doubt, throw it out. A new 30 mL vial of bacteriostatic water costs only a few dollars — far less than a contaminated reconstitution.

Storage best practice

• Store sealed vials at controlled room temperature (15–30°C / 59–86°F).
• Keep away from direct sunlight, freezing temperatures, and heat sources.
• After first puncture, label the vial with the open date so you can track the 28-day window.
• Always wipe the rubber stopper with a fresh alcohol swab before each draw.
• Use a new sterile needle for every withdrawal — never reuse needles in BAC water.

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