Bacteriostatic Versus Sterile Water for Injection

Bacteriostatic Water for Injection, USP, is a sterile, nonpyrogenic preparation of water containing 0.9% benzyl alcohol, 9 mg/mL, added as a bacteriostatic preservative, with a pH around 5.7. It is supplied in a multiple-dose container, and the preservative is the reason: it is what permits repeated withdrawals from one vial over time.

Sterile Water for Injection, USP, contains no preservative and is specified for single use, because once the seal is broken there is nothing in it to hold back microbial growth.

For bench work, that difference drives the choice. Where a preparation will be accessed more than once across days, the bacteriostatic grade is the one designed for it. Where a single withdrawal is planned, or where benzyl alcohol is unwanted in the preparation, the preservative-free grade is the reference point. Benzyl alcohol is not inert, and the USP labelling carries specific restrictions on its use in neonates.

This article describes the two diluents and how they differ. It is not guidance for administration. For working out reconstitution volumes, use the Peptheos Reconstitution Calculator.