Sterile water for injection is purified, sterilised water with no preservative and no added buffer salts. Used as a reconstitution solvent it produces a preservative-free solution, which means nothing suppresses microbial growth if an organism is introduced when the stopper is pierced. The practical consequence is that a vial reconstituted with sterile water is treated as single-entry and prepared fresh.
When it is the right choice
Preservative-free solvent is preferred where benzyl alcohol would confound the work: cell-culture and receptor-binding assays sensitive to alcohol, formulations where the preservative is known to promote aggregation, and any preparation intended to be aliquoted and frozen immediately. It is also the correct base when a defined buffer is going to be built rather than a ready-to-draw solution.
Sterile water versus bacteriostatic water
The choice is about in-use time, not potency. Bacteriostatic water supports repeated entry over a working period; sterile water does not. Concentration arithmetic is identical either way — a 5 mg vial in 2 mL is 2.5 mg/mL in both.
Why it matters
Choosing preservative-free solvent and then storing the vial for weeks is a common handling error. If a solution must last, either preserve it or split it into single-use aliquots and freeze them.
Related terms
stability · freeze–thaw. See the full comparison and Sterile Water for Reconstitution.