Bacteriostatic Water

Two ingredients. Water for injection, and benzyl alcohol as a bacteriostatic preservative. Bacteriostatic means it stops bacteria multiplying rather than killing what is already there, which is the property that matters for a vial you will go back into repeatedly.

Sources: Bacteriostatic Water for Injection, USP. Hospira, Inc. FDA-approved package insert, via DailyMed. and Bacteriostatic Water for Injection. USP-NF monograph M88830. Both read 2026-09-20.

The benzyl alcohol is 0.9% in plastic vials and 1.1% in glass vials. That is 9 mg per mL against 11 mg per mL, a difference of about a fifth.

It is a small difference and it almost never matters at the volumes used for reconstituting a vial. It is worth knowing only because the two strengths sit on shelves next to each other and the figure on the label is the one that applies.

The label carries NOT FOR USE IN NEONATES in boldface capitals, and the USP monograph requires that wording. The reason is gasping syndrome, a benzyl alcohol toxicity documented in neonatal intensive care in the early 1980s, presenting as central nervous system depression, metabolic acidosis and gasping respirations.

FDA labelling associates the syndrome with benzyl alcohol above 99 mg/kg/day, while stating plainly that the minimum amount at which toxicity may occur is not known. Both halves of that matter: there is a figure, and the label declines to treat it as a safe threshold.

For an adult the label offers a reference point rather than a warning: an estimated intravenous dose of up to 30 mL may be given without toxic effects, which carries about 270 mg of benzyl alcohol. Reconstituting a vial uses a fraction of that, so it is context rather than a constraint most people will meet.

Source: Bacteriostatic Water for Injection, USP. Hospira, Inc. FDA-approved package insert, via DailyMed. Read 2026-09-20.

The label warns that giving bacteriostatic water intravenously without a solute in it can cause haemolysis, because it is not isotonic. It exists to dissolve something, and once something is dissolved in it the resulting solution is what gets given.

That is also why the comparison people usually want is with sterile water rather than with saline, and it comes down entirely to the preservative. The comparison page works through it, the shelf life page covers how long a punctured vial is good for, and how much to add is a question about your syringe rather than about the water.

What is bacteriostatic water?

Sterile water for injection with benzyl alcohol added as a bacteriostatic preservative. The preservative stops bacterial growth in the vial, which is what allows the same vial to be entered more than once rather than being discarded after a single use.

What is in bacteriostatic water?

Water for injection and benzyl alcohol, and nothing else. The benzyl alcohol is 0.9 percent, or 9 mg per mL, in plastic vials, and 1.1 percent, or 11 mg per mL, in glass ones.

Is bacteriostatic water the same as saline?

No. Saline is water with sodium chloride in it and has no preservative unless it is specifically a bacteriostatic saline. Bacteriostatic water has the preservative and no salt, which is why the label warns that giving it intravenously on its own can cause haemolysis. It is a vehicle for dissolving something, not a fluid in its own right.

Why is bacteriostatic water not for newborns?

Benzyl alcohol has been associated with a fatal toxicity in neonates known as gasping syndrome. The label carries the warning NOT FOR USE IN NEONATES in boldface capitals for that reason, and preservative-free sterile water is used instead where a neonate is involved.

How much bacteriostatic water is safe?

The label gives a reference point rather than a limit: an estimated intravenous dose of up to 30 mL may be given to an adult without toxic effects from the benzyl alcohol, which at 9 mg/mL works out at about 270 mg. The equivalent figure for a 6 kg infant is 9 mL.