Sterile Water

Sterile water for injection is purified water that has been sterilised and contains neither a preservative nor any buffer salts. As a reconstitution solvent it gives a preservative-free solution — so if a microorganism gets in when the stopper is punctured, nothing holds its growth back. In practice, a vial made up with sterile water is therefore handled as single-entry and prepared freshly.

When to choose it

A solvent without preservative is the better option whenever benzyl alcohol would distort the results: cell-culture and receptor-binding assays that react to alcohol, formulations in which the preservative is known to encourage aggregation, and any preparation that will be divided into aliquots and frozen straight away. It is also the right starting point when you plan to build a defined buffer rather than a solution ready for drawing up.

Compared with bacteriostatic water

The decision concerns how long the vial can be used, not how potent the solution is. Bacteriostatic water allows repeated access during a working period; sterile water does not. The concentration maths does not change — a 5 mg vial in 2 mL is 2.5 mg/mL whichever solvent you use.

A frequent mistake

Picking a preservative-free solvent and then keeping the vial for weeks is a common handling error. If a solution needs to last, either use a preserved solvent or divide it into single-use aliquots and freeze them.

Related terms

stability · freeze–thaw. See the full comparison and Sterile Water for Reconstitution.