Bacteriostatic water, also known as BAC water, is a sterile solution used primarily for reconstituting freeze-dried powdered drugs and peptides intended for injectable use. Unlike traditional water for injection, bacteriostatic water contains 0.9% benzyl alcohol, a substance with a bacteriostatic effect that inhibits the growth of bacteria once the vial has been punctured.

This characteristic makesBAC water particularly useful when a reconstituted solution must be used in multiple administrations over time, as it allows for safer storage of the product, generally up to 14–28 days if properly refrigerated.Bacteriostatic water is widely used in the preparation of peptides, hormones, and other sensitive compounds, as it helps preserve their stability, reducing the risk of contamination and waste.

Thanks to its practicality and greater safety margin compared to simple sterile water, BAC water has become a benchmark for those who use injectable products that require precision, hygiene, and reliability in preparation.