Laboratory Guide

What Is Bacteriostatic Water?

Bacteriostatic water is sterile water with 0.9% benzyl alcohol: what the preservative does, how it differs from sterile water, and where its limits lie.

Updated 29 September 2026

[Research Use Only] This guide is for controlled laboratory research workflows only. It is not for human or veterinary use and does not provide applied-use guidance.

Sterile water with one addition

Bacteriostatic water is sterile, non-pyrogenic water for injection to which benzyl alcohol has been added as a preservative at 0.9%, or 9 mg per mL; one pharmaceutical label also lists a 1.1% presentation. One widely distributed product states its pH as 5.7, within a range of 4.5 to 7.0. It is supplied in multiple-dose containers, from which repeated withdrawals can be made. Set beside plain sterile water, the purpose of the addition becomes clear.

	Sterile water for injection	Bacteriostatic water for injection
Contents	Water for injection, with no bacteriostat, antimicrobial agent or added buffer	Water for injection with benzyl alcohol as a preservative
Container	Single-dose only	Multiple-dose
After first entry	Unused portion discarded; nothing in it restrains organisms introduced	The preservative restrains organisms introduced by repeated entry
Storage before opening	20–25 °C	20–25 °C

Both are sterile while sealed. The difference begins at the first entry, which is the only moment a preservative has any work to do.

What bacteriostatic means

Bacteriostatic describes an agent that holds bacteria back rather than one that sterilises, and pharmacopoeial testing makes the distinction measurable. In the antimicrobial effectiveness test the pharmacopoeias describe, a preserved multiple-dose product is inoculated with 100,000 to 1,000,000 colony-forming units per mL of each challenge organism — five in the USP — then held at 20–25 °C and sampled over 28 days. Under USP criteria, bacteria must fall by at least 1 log, or 90%, by day 7 and by 3 logs, or 99.9%, by day 14, then not increase; yeasts and moulds need only not increase. The European Pharmacopoeia is stricter, setting reductions at 6 and 24 hours.

Multiple-dose use in a laboratory

The preservative exists for one situation: a container that will be entered more than once. Each entry is a chance to introduce organisms, and the benzyl alcohol limits what they can do afterwards. It does not make the entries themselves clean, and it does nothing about contamination introduced when a solution is prepared.

Enter the container only under aseptic technique, which the pharmaceutical label requires for every entry and withdrawal
Record the date of first entry on the container and in the laboratory record, beside the batch number
Set the period of use by documented procedure; one widely distributed pharmaceutical label states storage conditions but no period of use after first entry
Do not use a solution that is not clear, or a container whose seal is not intact — the label’s own condition

Storage and shelf life

Sealed, bacteriostatic water is a room-temperature product: the pharmaceutical labels for it and for plain sterile water both specify 20–25 °C. The expiry date on the container applies to the container unopened. Once it has been entered, the limit is no longer the water’s chemistry but its microbiological history, which is why the in-use period is set by procedure rather than printed as a property of the product.

Where it falls short

Benzyl alcohol is useful because it is active against microorganisms, and that activity is why it is not neutral towards everything else in a solution.

Proteins and peptides: antimicrobial preservatives, benzyl alcohol among them, can promote partial unfolding and aggregation
Conditions matter: in one study, 0.9% benzyl alcohol accelerated aggregation of a recombinant protein at pH 7.0, far more at 37 °C than at 25 °C, and not at all at pH 3.5
Compatibility: the pharmaceutical label warns that some substances may be incompatible in a vehicle containing benzyl alcohol
Microbiological testing: because it can kill some contaminating cells, it can mask the true level of contamination, and FDA guidance calls for sterile water for injection instead when products are prepared for sterility testing

Research Checklist

Confirm batch identity and records.
Document all preparation inputs.
Keep use within controlled laboratory workflows.
Do not infer applied-use suitability from guide content.

Frequently Asked Questions

Is bacteriostatic water sterile?

Yes, as supplied, like sterile water for injection. What the benzyl alcohol adds is restraint on organisms introduced after the container is opened. It does not sterilise a solution that is already contaminated.

What is the difference between bacteriostatic water and sterile water?

Sterile water for injection contains no antimicrobial agent and is supplied only in single-dose containers. Bacteriostatic water contains benzyl alcohol as a preservative and is supplied in multiple-dose containers, so that it can be entered more than once.

How long does bacteriostatic water last once opened?

No single figure belongs to the water itself. The expiry date applies to the sealed container, and one widely distributed pharmaceutical label gives no period of use after first entry. In a laboratory the in-use period is a procedural decision, recorded against the date of first entry.

Is bacteriostatic water suitable for every peptide?

Not automatically. Benzyl alcohol has been shown to promote aggregation of some proteins, depending on pH and temperature, and the pharmaceutical label warns that some substances are incompatible with it. Suitability belongs to the material being dissolved, so its documentation is the place to look.

Sources

The technical statements in this guide are drawn from the following. Where a definition is contested or a figure depends on method, the guide says so rather than picking one.

Bacteriostatic Water for Injection, USP (Hospira): prescribing informationDailyMed, National Library of MedicineUsed for the composition — sterile, non-pyrogenic water for injection with 0.9% (9 mg/mL) or 1.1% (11 mg/mL) benzyl alcohol as a bacteriostatic preservative — the pH of 5.7 (4.5 to 7.0), multiple-dose containers, storage at 20–25 °C, aseptic technique for every entry, the clear-solution and intact-seal condition, the incompatibility warning, and the absence of a stated period of use after first entry.
Sterile Water for Injection, USP (Hospira): prescribing informationDailyMed, National Library of MedicineUsed for sterile water containing no bacteriostat, antimicrobial agent or added buffer, supply in single-dose containers only, discarding of the unused portion, and storage at 20–25 °C.
Comparison of Compendial Antimicrobial Effectiveness Tests: A ReviewAAPS PharmSciTech, via PMC, National Library of MedicineUsed for the antimicrobial effectiveness test for sterile multiple-dose products: the 10⁵ to 10⁶ CFU/mL challenge, five challenge organisms, storage at 20–25 °C over 28 days, the USP log-reduction criteria for bacteria and fungi, and the European Pharmacopoeia’s 6- and 24-hour criteria.
Lyophilization of Parenteral (7/93): Guide to InspectionsU.S. Food and Drug AdministrationUsed for bacteriostatic water’s capacity to kill some vegetative contaminants and mask the true level of contamination, and for the use of sterile water for injection when products are reconstituted for sterility testing.
Effect of antimicrobial preservatives on partial protein unfolding and aggregationJournal of Pharmaceutical Sciences, via PMC, National Library of MedicineUsed for antimicrobial preservatives, benzyl alcohol among them, inducing partial unfolding and aggregation of a model protein.
Effects of pH, temperature, and sucrose on benzyl alcohol-induced aggregation of recombinant human granulocyte colony stimulating factor (Thirumangalathu R et al., 2006; PMID 16729274)Journal of Pharmaceutical Sciences, via PubMedUsed for 0.9% benzyl alcohol accelerating aggregation of a recombinant protein at pH 7.0, much more at 37 °C than at 25 °C, and not inducing aggregation at pH 3.5.