Bacteriostatic water explained: what the preservative does and does not do

What the monograph and the label say

The title of the monograph is Bacteriostatic Water for Injection. The United States Pharmacopeia (USP) defines the article as prepared from the grade of water named in that title, sterilized and suitably packaged, containing one or more suitable antimicrobial agents.¹ The definition names no particular preservative.¹ Only that public definition was read for this article.

The label of a licensed product supplies the detail. Hospira’s product is described as a sterile, nonpyrogenic preparation containing 0.9% (9 mg/mL) or 1.1% (11 mg/mL) of benzyl alcohol, added as a bacteriostatic preservative.² Nonpyrogenic means not fever-causing. The pH is given as 5.7, within a range of 4.5 to 7.0, and the container is one from which, the label says, repeated withdrawals may be made.²

In Canada, Health Canada’s Drug Product Database lists a product of the same pharmacopoeial name in 30 mL vials, from Pfizer Canada under DIN 00038202, with the status “Marketed” when it was checked on 8 October 2026.⁷

The bacteriostatic water in this catalogue is listed as a laboratory supply for research use only. This article describes the pharmacopoeial article and one licensed product’s labelling, and makes no statement about how any other product compares with them.

What the preservative does

“Bacteriostatic” describes an agent that holds back the growth of bacteria, and that is the function the label gives for benzyl alcohol.²

Review A review of preservatives in licensed products found phenol and benzyl alcohol to be the two most common in peptide and protein products, and named three criteria for choosing one: the amount needed, its antimicrobial function and its effect on the active ingredient.⁸

Review Whether a preservative works is settled by a test. The antimicrobial effectiveness test is described in the three major pharmacopoeias for sterile products that are entered more than once; a review found the way it is run essentially harmonized among them, but not the acceptance criteria.⁹

Cell study The effect depends on concentration. When a protein medicine was mixed with saline preserved with benzyl alcohol, the mixture holding 0.54% benzyl alcohol met the USP criteria for a preserved solution in both batches tested, and the mixture holding 0.45% failed in one batch.¹⁰ Anything that dilutes preserved water dilutes its preservative.

What it does not do

• It does not sterilize, and it does not replace clean handling. The United States Centers for Disease Control and Prevention states that the preservative in a vial meant for repeated use has no effect on viruses and does not give complete protection against bacterial contamination.³ The label calls for aseptic technique at every entry.²
• It does not keep a dissolved substance stable. The label directs that solutions made with the product are not to be stored unless the maker of the dissolved substance says otherwise.² Chemical change in solution is covered in peptide stability and degradation.
• Chemistry study It is not chemically neutral. The label warns that some substances may be incompatible with a vehicle containing benzyl alcohol.² In a cell-free experiment, more of a freeze-dried protein aggregated when it was dissolved in water containing 0.9% benzyl alcohol than when plain water was used.¹¹ The result depends on the molecule: benzyl alcohol did not induce aggregation of an acylated 31-residue peptide, while another preservative, m-cresol, produced insoluble aggregates within a day.¹²

How it differs from sterile water

Both articles are prepared from the same grade of water, sterilized and suitably packaged.¹⁴ The table sets the definitions and the two Hospira labels side by side.²¹³

The chemistry of dissolving a dry peptide in either is the subject of reconstitution concepts.

Why the preservative matters in cell-based assays

The preservative travels with the water into every dilution. Benzyl alcohol has a molecular weight of 108.14 g/mol, so 9 mg/mL is about 83 millimoles per litre.¹⁴ A stock made in preserved water and diluted tenfold into culture medium leaves 0.9 mg/mL, about 8 mM; a hundredfold dilution leaves 0.09 mg/mL. The arithmetic is set out in peptide units and concentration.

Cell study Concentrations of that order have measurable effects on cultured cells. Human and rabbit retinal pigment epithelial cells in culture were exposed to benzyl alcohol at 0.0225 to 9 mg/mL for between five minutes and two hours, and the damage rose with concentration and with time.⁵ At 9 mg/mL it was toxic within five minutes in every assay; at 0.225 mg/mL it caused ultrastructural damage and impaired cell function at two hours; at 0.0225 mg/mL it did not.⁵

Cell study Benzyl alcohol can also shift what an assay measures. In cultured kidney epithelial cells it made the membrane more fluid as its concentration rose, and it doubled the resting content of cyclic AMP.⁶ At 10 mM it increased the cyclic AMP response to three stimulants, and at 40 mM it inhibited the response to three.⁶

Where an assay reads out cyclic AMP, that matters: 10 mM is close to what a tenfold dilution of preserved water leaves behind. This is the case for a vehicle control, a sample that carries the same final concentration of benzyl alcohol without the peptide.

Storage and the opened container

The label gives a storage temperature of 20 to 25 °C and, as read for this article, no period of use after first opening.² A 28-day figure comes from USP General Chapter 797, as summarized by the Centers for Disease Control and Prevention: a vial meant for repeated use should be dated when it is opened and discarded within 28 days unless the manufacturer states another date, and never later than the manufacturer’s expiry date.³ That guidance is written for health-care settings. General conditions for peptides, dry and dissolved, are in the peptide storage guide.

Human evidence

Human study The human evidence cited here concerns harm in newborn infants. The label carries the warning “Not for use in neonates” and attributes it to the potential toxicity of benzyl alcohol in that group.² A 1986 study reviewed the records of one neonatal intensive care unit for the 13 months before and the 13 months after solutions containing benzyl alcohol were withdrawn.¹⁵ Among infants born weighing less than 1,000 g, mortality was 80.7% in those who received the preservative and 45.7% in those who did not, and severe bleeding into the ventricles of the brain occurred in 46% and 19%.¹⁵ The comparison was retrospective, from one unit; its authors concluded that benzyl alcohol toxicity had contributed to both outcomes.¹⁵

Everything else on this page comes from official documents and laboratory studies. Nothing here addresses use in people.