Bacteriostatic Water

Bacteriostatic water — abbreviated BAC water — is sterile water for injection containing 0.9% benzyl alcohol as a preservative. The benzyl alcohol prevents bacterial growth in the vial after the stopper has been pierced, which is what allows a multi-dose research vial to be used over multiple days or weeks.

BAC water is the default diluent for most peptide research because reconstitution typically targets multi-dose vials (e.g., 5 mg of peptide reconstituted in 2 mL, used over 14–30 days). Plain sterile water for injection — without preservative — is intended for single-use only and is appropriate for single-dose research preparations.

BAC water is not appropriate for neonatal use clinically due to benzyl-alcohol toxicity in that population, which is why some clinical contexts mandate plain sterile water. For adult research-context use, BAC water is the standard.