What Is Bacteriostatic Water? Composition and Lab Use

Bacteriostatic water is sterile, nonpyrogenic water for injection that contains 0.9% benzyl alcohol (9 mg/mL) as a preservative. The preservative inhibits bacterial growth, so the same vial can be entered repeatedly to dilute or dissolve powdered substances. In research laboratories it is the most common solvent for reconstituting lyophilized peptides.

For laboratory research use only. The compounds discussed are not approved for human or veterinary use and are not drugs, foods or cosmetics. This article summarises published research and is not medical advice.

Key points

Composition: water for injection + benzyl alcohol (0.9%, sometimes 1.1%). Nothing else.
“Bacteriostatic” means growth-inhibiting, not sterilising.
Supplied in multiple-dose vials; unbuffered, pH about 5.7 (range 4.5–7.0).
Hypotonic: the label warns of hemolysis if used intravenously without an added solute.
Contraindicated in neonates because of benzyl alcohol toxicity.

Composition and specifications

Property	Value (U.S. prescribing information)
Base	Water for Injection, USP (H₂O)
Preservative	Benzyl alcohol 0.9% (9 mg/mL) or 1.1% (11 mg/mL)
pH	5.7 (4.5–7.0); no buffer
Sterility / pyrogens	Sterile, nonpyrogenic
Container	Multiple-dose vial; repeated withdrawals permitted
Storage (unopened)	20–25 °C
Tonicity	Hypotonic

The preservative, benzyl alcohol (C₆H₅CH₂OH; 108.14 g/mol; PubChem CID 244), is a simple aromatic alcohol. At 9 mg/mL it is well below its water solubility, so the solution is clear and colourless.

How benzyl alcohol works

Each time a needle passes through a rubber stopper, a few microorganisms may be carried in. In unpreserved water they can multiply over the following days. Benzyl alcohol disrupts microbial membranes and inhibits growth, which keeps the bioburden low between withdrawals. It does not reliably kill a heavy contamination, which is why aseptic technique and an in-use time limit still apply (see does bacteriostatic water need to be refrigerated?).

The preservative’s antimicrobial efficiency is not fixed. A Roche study tested a model peptide with benzyl alcohol, phenol and m-cresol and found that preservative molecules interacted with the peptide. This affected both peptide self-association and the preservative’s performance in the Ph. Eur. antimicrobial efficacy test (Heljo et al., 2015).

Bacteriostatic water compared with other waters

Type	Sterile	Pyrogen-free	Preservative	Multi-use
Bacteriostatic water for injection	Yes	Yes	Benzyl alcohol	Yes
Sterile water for injection	Yes	Yes	None	No (single-dose)
Bacteriostatic 0.9% sodium chloride	Yes	Yes	Benzyl alcohol	Yes; isotonic
Distilled / deionized water	Not necessarily	Not necessarily	None	Not designed for it

For a detailed comparison of the first two, see bacteriostatic water vs sterile water. Distilled and deionized water are laboratory-grade purified waters. They are not manufactured or tested to injectable standards and should not be substituted when sterility matters.

Label restrictions and their origin

Two warnings on the label come from well-documented events:

Neonates: in 1982, deaths among very low birth-weight infants were linked to “gasping syndrome” caused by benzyl alcohol in preserved flush solutions (Gershanik et al., 1982; CDC, 1982). Labels now contraindicate benzyl-alcohol-containing solutions in this group.
Hemolysis: plain water is hypotonic. The label warns against intravenous use unless enough solute is added to make the solution approximately isotonic.

The label also warns that some drugs may be incompatible with a vehicle containing benzyl alcohol. For peptides this is a real consideration: formulation work at Merck used NMR to show how antimicrobial excipients can drive aggregation of an acylated peptide (Li et al., 2022).

How it is used in peptide research

In the laboratory, bacteriostatic water serves as the default solvent when one vial of reconstituted peptide will be sampled several times. It is typically added slowly down the side of the vial and swirled gently until the powder dissolves. The procedure is in how to reconstitute peptides, and the choice of solvent for specific peptides is discussed in bacteriostatic water for peptide reconstitution. To work out the resulting concentration, use the peptide reconstitution calculator.

We supply bacteriostatic water (0.9% benzyl alcohol) in 3 mL and 10 mL vials for laboratory use.

Related research

Frequently asked questions

What is bacteriostatic water made of?

Two ingredients: water for injection (USP) and benzyl alcohol, usually at 0.9% (9 mg/mL). There is no buffer, salt or other additive. Some products contain 1.1% benzyl alcohol.

What is bacteriostatic water used for?

Its labelled use is diluting or dissolving drugs supplied as powders or concentrates. In research laboratories it is widely used to reconstitute lyophilized peptides when a vial will be sampled more than once.

What does bacteriostatic mean?

Bacteriostatic means the agent inhibits bacterial growth and reproduction rather than reliably killing all bacteria (bactericidal). Benzyl alcohol keeps contamination from multiplying between withdrawals; it does not sterilise a contaminated vial.

Is bacteriostatic water the same as distilled or deionized water?

No. Distilled and deionized water are purified but not necessarily sterile or pyrogen-free, and contain no preservative. Bacteriostatic water is manufactured as sterile, nonpyrogenic water for injection with benzyl alcohol added.

Why can't bacteriostatic water be used in newborns?

Benzyl alcohol was linked in 1982 to fatal ‘gasping syndrome’ in very low birth-weight infants given preserved flush solutions. Current labels contraindicate preserved solutions in neonates for this reason.

References

DailyMed. Bacteriostatic Water (water) injection, solution — prescribing information. U.S. National Library of Medicine. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=a833502e-a622-4e81-8c58-cef6ccb6c4c9
PubChem Compound Summary for CID 244, Benzyl alcohol. National Center for Biotechnology Information. https://pubchem.ncbi.nlm.nih.gov/compound/244
Gershanik J, et al. The gasping syndrome and benzyl alcohol poisoning. N Engl J Med. 1982. PMID: 7133084 https://pubmed.ncbi.nlm.nih.gov/7133084/
Centers for Disease Control (CDC). Neonatal deaths associated with use of benzyl alcohol–United States. MMWR Morb Mortal Wkly Rep. 1982. PMID: 6810084 https://pubmed.ncbi.nlm.nih.gov/6810084/
Heljo P, et al. Interactions Between Peptide and Preservatives: Effects on Peptide Self-Interactions and Antimicrobial Efficiency In Aqueous Multi-Dose Formulations. Pharm Res. 2015. PMID: 25893328 https://pubmed.ncbi.nlm.nih.gov/25893328/
Li M, et al. Molecular Mechanism of Antimicrobial Excipient-Induced Aggregation in Parenteral Formulations of Peptide Therapeutics. Mol Pharm. 2022. PMID: 35917158 https://pubmed.ncbi.nlm.nih.gov/35917158/