Bacteriostatic Water vs. Sterile Water: What's the Difference?

Both products start from the same base: water that has been sterilized and verified non-pyrogenic. The difference is a single ingredient — 0.9% benzyl alcohol — and that one ingredient changes how each product is used in the laboratory.

Sterile water: single-use by design

Sterile water contains no preservative. Inside a sealed vial it is perfectly sterile, but the moment the seal is broken there is nothing to suppress microbial growth. For that reason laboratory convention treats an opened vial of plain sterile water as single-use: draw what is needed, then discard the vial.

Bacteriostatic water: built for repeated access

Bacteriostatic water — often shortened to bac water — adds 0.9% benzyl alcohol as a bacteriostatic preservative. "Bacteriostatic" means the additive does not sterilize the contents after contamination; it inhibits bacteria from multiplying inside the vial. That protection is what allows a vial to be punctured and drawn from more than once over a working period, following the laboratory's own SOP.

Which one to choose

• The vial will be accessed once: either product works; plain sterile water is the simpler choice.
• The vial will be accessed repeatedly: bacteriostatic water is the appropriate choice — it is the only one of the two designed for multiple withdrawals.
• The protocol prohibits preservatives: some assays are sensitive to benzyl alcohol; in that case use plain sterile water and treat every vial as single-use.

Quality still matters

Preservative or not, the water must be sterile, non-pyrogenic and USP grade, and each lot should be verifiable. Every vial we supply ships with a third-party certificate of analysis you can check in our public certificate library before ordering.

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