What is bacteriostatic water?

Bacteriostatic water is sterile water containing a preservative — commonly 0.9% benzyl alcohol — which inhibits bacterial growth and allows a vial to be accessed more than once.

Bacteriostatic, not bactericidal

The distinction is in the name and it matters. Benzyl alcohol suppresses bacterial reproduction; it does not sterilise the solution or kill an established contamination. It buys a usable window, not immunity from poor technique.

How it differs from the alternatives

• Sterile Water for Injection (SWFI) contains no preservative. It is intended for single use, because nothing stops growth once the seal is broken.
• Bacteriostatic Water for Injection (BWFI) contains the preservative and supports multi-dose access. Clinical convention discards multi-dose vials 28 days after first puncture.
• Saline is isotonic and used for different purposes; it is not interchangeable in reconstitution protocols that specify BWFI.

Why it is the usual diluent

Peptides are reconstituted and then drawn from over days or weeks, so the preservative is what makes that workable — and it is the reason a reconstituted vial is often given a 14–30 day refrigerated window.

Note that benzyl alcohol is not appropriate in every context; some populations and some routes call for preservative-free diluents.

See what liquid to reconstitute with and the calculator. Research use only. Not medical advice.