Sterile water with one addition
Bacteriostatic water is sterile water for injection with a small amount of benzyl alcohol added, typically around 0.9%. That single addition is what distinguishes it from plain sterile water and gives it its name.
What “bacteriostatic” means here
Bacteriostatic describes something that inhibits bacterial growth without necessarily killing bacteria outright, as opposed to “bactericidal,” which kills them. The benzyl alcohol content slows bacterial growth in a multi-dose vial across repeated punctures over its usable life.
Why it’s preferred for multi-dose reconstitution
A vial that’s accessed multiple times, once per dose over several weeks, carries more contamination risk than a single-use vial. Bacteriostatic water’s antimicrobial property is specifically useful in that repeated-access scenario, which is exactly how most reconstituted peptide vials are used.
When plain sterile water is used instead
Single-use, immediate-use preparations sometimes use plain sterile water instead, since the antimicrobial property matters less when the entire vial is used at once rather than accessed repeatedly over weeks.
A note on benzyl alcohol
Because bacteriostatic water contains benzyl alcohol, it’s generally not recommended for certain populations (including neonates, in clinical contexts) where benzyl alcohol exposure carries specific risks. This is a general pharmacology note, not guidance for any individual’s situation.