Bacteriostatic water
CompoundGuide Editorial · Updated Oct 14, 2026 · Research reference, not medical advice.
Bacteriostatic water for injection is sterile water with benzyl alcohol added as a preservative, supplied in a multiple-dose vial so a drug can be diluted or dissolved for injection. The Hospira label specifies 0.9% (9 mg/mL) or 1.1% benzyl alcohol, and states it is not for use in neonates.
What the label actually says
The Hospira DailyMed label calls Bacteriostatic Water for Injection, USP a sterile, nonpyrogenic preparation of water for injection. Benzyl alcohol is the bacteriostatic preservative. Two strengths are on that label: 0.9% (9 mg/mL) and 1.1% (11 mg/mL). It is filled in a multiple-dose container so repeated withdrawals can dilute or dissolve drugs for injection. The labeled pH is 5.7, with a range of 4.5 to 7.0.
The indication is narrow. The label says the preparation is for parenteral use only after addition of drugs that require dilution or must be dissolved in an aqueous vehicle, and only for intravenous, intramuscular, or subcutaneous injection according to the instructions of the drug manufacturer. It is a diluent. It is not a treatment.
What it is not
It is not preservative-free sterile water. The label is explicit that solutions containing benzyl alcohol must not be used in neonates, and that where water is required to prepare or dilute medications for neonates, only preservative-free Sterile Water for Injection should be used. It is not a fluid-replacement product. It must be made approximately isotonic before use. Intravenous injection of bacteriostatic water without a solute may cause hemolysis.
It is also not a storage system for a reconstituted peptide. The same label says not to store reconstituted solutions of drugs for injection unless the manufacturer of the solute directs otherwise, and to inspect the reconstituted drug for clarity and unexpected precipitation or discoloration before administration.
Why the term shows up next to peptides
Research peptides are usually shipped as a dry solid. Turning that solid back into a solution requires a diluent. In peptide forums and vendor pages, "BAC water" is the shorthand for this labeled product. The shorthand hides the constraints: benzyl alcohol concentration, the neonatal warning, the isotonicity requirement, and the instruction not to keep a reconstituted solution just because the vial still has liquid in it.
CompoundGuide uses the term on the reconstitution and storage guides. Those pages cover handling. This entry only defines the diluent. It does not set a volume, a concentration, or a schedule.
How to read a vial
Match the label, not the nickname. A vial marked bacteriostatic water should state the benzyl alcohol percentage and the "not for use in neonates" warning. A vial of sterile water without that preservative is a different product, even if both are clear and both are used to dissolve a powder. If the seal is broken, the solution is not clear, or the solute manufacturer forbids storage, the label's own instructions stop the process there.