Bacteriostatic water vs sterile water — what's the difference

Bacteriostatic water and sterile water are not interchangeable. They look the same in the vial, but one contains a preservative and one does not. That single difference changes shelf life, vial size, and which medications you can use it with.

The core difference

Bacteriostatic water contains 0.9% benzyl alcohol as a preservative. Sterile water contains no preservative at all. The benzyl alcohol inhibits bacterial growth, which is why bacteriostatic water can be punctured multiple times over a 28-day window. Sterile water has no such protection — once the vial is punctured, it must be used immediately and discarded.

Comparison

When to use bacteriostatic water

Use bacteriostatic water any time you need multiple withdrawals from the same vial over a period of days or weeks. The benzyl alcohol preservative is what makes multi-dose use possible.

When to use sterile water

Use sterile water when the medication or protocol specifically requires a preservative-free diluent. Certain intrathecal, epidural, and neonatal applications require preservative-free water because benzyl alcohol can cause adverse reactions in those settings.

Important safety note

Do not use bacteriostatic water in newborns or premature infants. Benzyl alcohol is toxic to neonates and has been associated with serious adverse events including gasping syndrome. For neonatal use, sterile preservative-free water is required.