Bacteriostatic water (often called “bac water”) is a sterile, non-pyrogenic water solution that contains 0.9% benzyl alcohol as a preservative to help prevent bacterial growth. It is commonly used to dilute or reconstitute lyophilized (freeze-dried) compounds before injection. Because of the preservative, it can typically be used multiple times for up to about 28 days after first puncture when handled and stored properly, unlike plain sterile water which is single-use. It is widely used in laboratory and clinical environments where repeated, sterile reconstitution is required.