What is bacteriostatic water (BAC water)?

What is bacteriostatic water?

Bacteriostatic water is sterile, nonpyrogenic water with benzyl alcohol added as a bacteriostatic preservative. Bacteriostatic describes a preservative that limits the growth of bacteria, and CDC says such preservatives give no complete protection against contamination (CDC, 26 March 2024). The US label gives the benzyl alcohol content as 0.9% or 1.1% and describes a multiple-dose container (DailyMed, read 14 October 2026). 'BAC water' is informal shorthand for the same product, and nonpyrogenic means free of fever-causing contaminants.

The full label name adds a route phrase that this page leaves out, because the page gives no directions for any route. The USP in the label name is the United States Pharmacopeia, the book of official drug-quality standards.

What does bacteriostatic water contain?

Bacteriostatic water contains water and benzyl alcohol. The label reviewed lists the benzyl alcohol content as 0.9% in its 30 mL plastic vials and 1.1% in its 20 mL glass vials, and describes the water component as chemically H2O (DailyMed, read 14 October 2026). Benzyl alcohol is an aromatic alcohol with the formula C7H8O, and the body metabolises it into benzoic acid (EMA, 9 October 2017). The label names benzyl alcohol as the one bacteriostatic agent and lists no salt, so bacteriostatic water is not saline.

Benzyl alcohol turns up in other products too. EMA says it is used as an excipient (an ingredient other than the active substance) for its preservative properties, as a solubilising agent or as a fragrance, and that it appears in medicines given into muscle, into veins and on the skin (EMA, Questions and answers on benzyl alcohol used as an excipient, EMA/CHMP/508188/2013, 9 October 2017).

Is bacteriostatic water sodium chloride?

No. Bacteriostatic water contains no sodium chloride, while saline is sodium chloride dissolved in water. The bacteriostatic sodium chloride label reviewed describes a sterile, isotonic (close to the particle concentration of body fluids) solution of sodium chloride in WFI that also contains 0.9% benzyl alcohol, and the plain 0.9% sodium chloride labels (second record) describe the same salt solution without a preservative, with an osmolarity of 308 mOsmol/L (DailyMed, read 14 October 2026). Saline is therefore salt water of a controlled strength made with sterile water, and '0.9 sodium chloride' is simply that strength written another way.

What pH is bacteriostatic water?

The six full DailyMed labels for bacteriostatic water (8 July 2026, 17 November 2025, 17 November 2025, 10 January 2025, 23 May 2023 and 7 July 2014), one from the manufacturer and five from repackagers, each state a pH of 5.7 within a range of 4.5 to 7.0 (read 14 October 2026). On the pH scale 7 is neutral, so 5.7 is mildly acidic. The figure is a product specification for the finished vial. The labels set no pH target for anything else, and no source read for this page gives one. The figure describes the water as supplied in the sealed vial, not any solution made later by adding something to it.

What do DailyMed labels say about bacteriostatic water?

DailyMed lists seven bacteriostatic water records as of 14 October 2026: six with full label text and one, dated 25 June 2026, that carries a supply description and the marketing category 'unapproved drug other'. The six full labels share one text. They carry a drug-diluent category and a neonate contraindication, warn that hemolysis may result when the water is given into a vein without a solute, and carry a room-temperature storage statement. The six records name marketing application NDA018802.

DailyMed is the US National Library of Medicine's database of drug labels. The regulatory status page sets out the US and Australian records with their dates.

How can you tell whether bacteriostatic water has gone bad?

No label read for this page describes a test for spoiled bacteriostatic water, and its vial label prints lot and expiry fields (DailyMed, read 14 October 2026). CDC says a vial preservative gives no complete protection against bacterial contamination (CDC, 26 March 2024). Contamination of opened multi-dose vials has been measured in hospital surveys of mixed medicines, none of which tested bacteriostatic water, and each result is reported separately below.

In a one-day survey at a 1300-bed hospital in November 2001, 1 vial and 1 spike among 227 opened multi-dose vials of mixed medicines were contaminated, only 114 of the vials had an opening date marked, and the survey did not test bacteriostatic water or measure infections (Mattner and Gastmeier, American Journal of Infection Control, 2004). In a four-month survey at one Iranian teaching hospital, 36 of 637 used multidose vials held bacteria, mostly organisms that normally live on people, and the authors wrote that the clinical significance was not resolved (Motamedifar and Askarian, American Journal of Infection Control, 2009).

In a two-month survey at one Iranian pulmonary teaching hospital, 11 of 205 opened single- and multiple-dose vials were contaminated, and the survey did not test bacteriostatic water (Baniasadi et al., Brazilian Journal of Infectious Diseases, 2013). In a cross-sectional study of 384 parenteral medications at one Ethiopian medical centre in 2021, 21 multiple-dose vials and none of the single-dose vials were contaminated, and the study did not test bacteriostatic water (Tabor et al., Infection Prevention in Practice, 2023). Each survey sampled mixed hospital stocks, so none describes bacteriostatic water.

Can old or contaminated bacteriostatic water make you sick?

Contaminated multi-dose vials have caused infections in documented outbreaks, but no outbreak report read for this page names bacteriostatic water. A 2019 outbreak analysis described acute hepatitis C in 12 patients who received a saline flush from one multi-dose vial after a CT scan, found that viral sequences from 7 of the patients clustered closely with the suspected source patient, and concerned saline, not bacteriostatic water (Shteyer et al., PLoS One, 2019). A 2013 outbreak report from one Cambodian hospital recorded Burkholderia cepacia bloodstream infections in eight adult patients, three of whom died, whose catheters were flushed from a Ringer lactate bag used as a multiple-dose vial, and concerned Ringer lactate, not bacteriostatic water (De Smet et al., Clinical Microbiology and Infection, 2013).

Whether an old vial carries more risk than a new one is a separate question that those reports do not answer. The US label's adverse-reaction list names infection at the entry site among reactions that may occur because of the solution, an added drug or the technique used (DailyMed, read 14 October 2026). How long bacteriostatic water lasts covers the dates.

Does bacteriostatic water sting or burn?

No study read for this page measured stinging or burning from bacteriostatic water itself. The cited pain trials examined bacteriostatic saline, a different product from bacteriostatic water. In a trial of 221 participants randomly assigned by lottery convenience sampling to lidocaine, bacteriostatic saline or no local anaesthetic, bacteriostatic saline was less painful to place than lidocaine and both were effective as local anaesthetics, and the trial tested bacteriostatic saline, not bacteriostatic water (Windle et al., Journal of Perianesthesia Nursing, 2006). In a randomised, double-blind study of 148 adults, buffered lidocaine reduced catheter pain significantly more than bacteriostatic saline (P = 0.007), and the study tested bacteriostatic saline, not bacteriostatic water (Burke et al., American Journal of Nursing, 2011).

Those findings concern a different product and a different question from water in a vial, so they do not show whether bacteriostatic water burns. What labels and studies report covers the pain studies in more detail.

Evidence limits

What has not been shown about bacteriostatic water?