What is the difference between bacteriostatic water and sterile water?

Bacteriostatic water has benzyl alcohol added as a preservative and sterile water does not. Both start as sterile water for injection. The preservative stops bacteria multiplying in the vial between uses, which is what allows the same vial to be entered repeatedly.

Can I use sterile water instead of bacteriostatic water?

For a single use, the reconstitution arithmetic is identical and the diluent behaves the same. The difference arrives at the second entry: sterile water has nothing stopping bacterial growth, so a vial reconstituted with it is a single-dose preparation and the remainder is discarded.

Is bacteriostatic water sterile?

Yes. It is sterile water with a preservative added, not an alternative to sterility. The preservative maintains that state through repeated entry rather than creating it.

Which one should I use?

Whichever the product being dissolved specifies. Where nothing specifies and the vial will be entered more than once, the preservative is the reason bacteriostatic water exists. Where a neonate is involved, the preservative is the reason it must not be used.

Bacteriostatic Water vs Sterile Water

They are the same liquid. One has a preservative in it, and that decides everything else.

Side by side

	Bacteriostatic water	Sterile water
Preservative	Benzyl alcohol, 0.9% or 1.1%	None
Container	Multiple-dose vial	Single-dose vial or ampoule
Repeat entry	Yes, that is what it is for	No, discard after one use
In-use period	None on the label; 28 days by USP <797>	Use immediately, discard the remainder
Neonates	NOT FOR USE IN NEONATES	The preservative-free option, where one is needed
Given alone intravenously	No, haemolysis risk	No, haemolysis risk

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

Why the preservative is the whole story

Both products start as water for injection and both are sterile when the stopper is intact. What changes is what happens after the first needle goes through it. A puncture is an opportunity for contamination, and without a preservative there is nothing to stop whatever gets in from multiplying between now and the next dose.

Benzyl alcohol is bacteriostatic rather than bactericidal, meaning it holds growth in check rather than sterilising anything. That is enough to make repeated entry defensible, and it is the reason a 28 day in-use period exists for one and not the other. The shelf life page covers where that figure comes from.

The one case where the preservative is the problem

Benzyl alcohol is why bacteriostatic water carries NOT FOR USE IN NEONATES in boldface capitals. Where a neonate is involved, preservative-free sterile water is the correct choice and the multi-dose convenience is simply not available.

For adults the label offers a reference figure rather than a prohibition: an estimated intravenous dose of up to 30 mL without toxic effects from the preservative. The bacteriostatic water page has the detail and the sources.

Questions

What is the difference between bacteriostatic water and sterile water?: Bacteriostatic water has benzyl alcohol added as a preservative and sterile water does not. Both start as sterile water for injection. The preservative stops bacteria multiplying in the vial between uses, which is what allows the same vial to be entered repeatedly.
Can I use sterile water instead of bacteriostatic water?: For a single use, the reconstitution arithmetic is identical and the diluent behaves the same. The difference arrives at the second entry: sterile water has nothing stopping bacterial growth, so a vial reconstituted with it is a single-dose preparation and the remainder is discarded.
Is bacteriostatic water sterile?: Yes. It is sterile water with a preservative added, not an alternative to sterility. The preservative maintains that state through repeated entry rather than creating it.
Which one should I use?: Whichever the product being dissolved specifies. Where nothing specifies and the vial will be entered more than once, the preservative is the reason bacteriostatic water exists. Where a neonate is involved, the preservative is the reason it must not be used.