Why Is Bacteriostatic Water Good for 28 Days After Puncture?

The moment any sterile liquid vial is punctured, it is exposed to the outside environment and carries some risk of microbial introduction with each subsequent draw. Plain sterile water has no defense against this, which is why it is generally treated as single-use only once opened.

Bacteriostatic water's benzyl alcohol content actively inhibits bacterial growth within the vial between uses, which is the specific property that extends its practical usable window to roughly 28 days post-puncture under refrigeration — not an indefinite window, but long enough to support a multi-withdrawal research workflow rather than requiring a fresh vial every time.

This 28-day reference applies to the bacteriostatic water itself as a diluent — it is separate from, and generally longer than, the shelf-life window of a peptide solution once that water has been used to reconstitute a specific compound.