Bacteriostatic Water: The Chemistry of a Preserved Sterile Diluent
18 Oct 2026Bacteriostatic Water: The Chemistry of a Preserved Sterile Diluent
Bacteriostatic Water for Injection USP is sterile, non-pyrogenic water containing 0.9% (9 mg/mL) benzyl alcohol as a bacteriostatic preservative. That one additive is the entire difference from Sterile Water for Injection USP, which the label states “contains no bacteriostat, antimicrobial agent or added buffer” and is “supplied only in single-dose containers.” The preservative is what makes a vial multiple-dose.
What is bacteriostatic water for injection?
Bacteriostatic Water for Injection USP is Water for Injection — chemically just H2O, purified to the pharmacopoeial standard for parenteral water and rendered non-pyrogenic — to which benzyl alcohol has been added as a bacteriostatic preservative. The Pfizer/Hospira labelling gives the preservative content as 0.9% (9 mg/mL) or 1.1% (11 mg/mL) depending on the presentation, and states the pH as 5.7 with a specification range of 4.5 to 7.0. The product we stock is the 0.9% presentation.
“Bacteriostatic” is a precise word, not a synonym for sterile. A bacteriostatic agent inhibits bacterial multiplication; a bactericidal agent kills. The water is sterile when it leaves the manufacturer because it was sterilised, not because benzyl alcohol made it so. The preservative's job starts afterwards: it suppresses growth of the few organisms a puncture may introduce, so the contents stay acceptable through repeated withdrawals rather than being compromised by the first. ↑ Back to top
How does it differ from Sterile Water for Injection USP?
The two products are frequently treated as interchangeable and are not. The difference is one ingredient, and that ingredient changes the container, the permitted number of entries and the in-use dating.
| Property | Bacteriostatic Water for Injection USP | Sterile Water for Injection USP |
|---|---|---|
| Preservative | Benzyl alcohol, 0.9% (9 mg/mL) or 1.1% (11 mg/mL) per label | None. Label states it “contains no bacteriostat, antimicrobial agent or added buffer” |
| Container type | Multiple-dose container; repeated withdrawals permitted | “Supplied only in single-dose containers” |
| Entries permitted | More than one, with aseptic technique | One. Discard the remainder |
| In-use dating after first puncture | Governed by the manufacturer's labelling; USP <797> default for multiple-dose containers is 28 days | Not applicable — single use |
| pH | 5.7 (specification 4.5–7.0) | Not stated in the DESCRIPTION section of the label reviewed |
| Antimicrobial effectiveness testing | Required — USP <51> must be demonstrated for all injections in multiple-dose containers | Not applicable — no preservative to test |
| Tonicity note | Water with a preservative; still not isotonic on its own | Label warns that intravenous administration without a solute may cause haemolysis |
| Neonatal use | Label directs that preservative-free Sterile Water be used instead for neonates | The preservative-free option the labelling points to |
What does benzyl alcohol actually do?
Benzyl alcohol is C6H5CH2OH, an aromatic alcohol with a benzene ring attached to a hydroxymethyl group. It is modestly soluble in water and freely miscible with organic solvents, and that amphiphilic character is the basis of its activity: the non-polar ring partitions into microbial membrane lipids, disordering the membrane and interfering with permeability and transport. Growth stops; the organism is not necessarily killed. Hence bacteriostatic.
The same partitioning explains both of its well-documented drawbacks, below. A molecule that inserts into lipid bilayers also interacts with hydrophobic surfaces on proteins, and a small aromatic alcohol distributed systemically has to be metabolised. Neither is a defect of a particular brand; both are consequences of the chemistry. ↑ Back to top
Why does the preservative permit multiple punctures of one vial?
Every puncture of an elastomeric closure is a microbiological event. The needle passes through the external surface of the stopper and the stopper reseals behind it; even with correct aseptic technique and surface disinfection, the probability of introducing organisms is not zero. In an unpreserved vial anything that gets in has nothing opposing it, which is why single-dose containers are specified for one entry with the remainder discarded.
A preserved vial changes that arithmetic, and the capacity is demonstrated rather than assumed. USP General Chapter <51>, Antimicrobial Effectiveness Testing, requires that preservative efficacy “must be demonstrated for all injections packaged in multiple-dose containers.” The product is challenged in its original sealed container with five organisms — Candida albicans (ATCC 10231), Aspergillus brasiliensis (ATCC 16404), Escherichia coli (ATCC 8739), Pseudomonas aeruginosa (ATCC 9027) and Staphylococcus aureus (ATCC 6538) — and judged on log reductions at 14 and 28 days against category-specific criteria. Injections fall in Category 1, the strictest.
What governs the in-use period after first puncture?
Three things, in order of precedence: the manufacturer's labelling for that specific product, the applicable pharmacopoeial or practice standard, and the container's own expiry date.
The widely used default comes from USP General Chapter <797>. As the CDC summarises it, an opened multi-dose vial should be dated and discarded within 28 days of first entry unless the manufacturer specifies a different period for that opened vial — and the assigned beyond-use date “should never exceed the manufacturer's original expiration date.” Our product pages state approximately 28 days after first access on that basis.
Two points deserve precision. The 28-day figure is a conservative administrative convention for preserved multiple-dose containers generally; it is not a measured stability result for any particular vial, and nothing happens to the preservative on day 29. And it is a limit on the container, dated from first puncture, not on anything prepared with the water. The stability of any solution a researcher prepares depends on the solute and is the researcher's to validate. See how to store research peptides. ↑ Back to top
What do sterility, particulates and closure integrity mean here?
Three separate quality attributes get collapsed into the word “sterile” on marketing pages. They are distinct and are assessed differently.
Sterility
USP General Chapter <71>, Sterility Tests, specifies membrane filtration or direct inoculation into Fluid Thioglycollate Medium and Soybean–Casein Digest Medium, incubated at least 14 days. The chapter is explicit about its own limits: “These Pharmacopeial procedures are not by themselves designed to ensure that a batch of product is sterile.” A pass means no contaminating organism was recovered from the sample tested; sterility assurance comes from a validated process, not the end-product test. The same distinction applies to a peptide certificate of analysis: a test on a sample is evidence about the sample.
Particulate matter
USP General Chapter <788>, Particulate Matter in Injections, covers “extraneous mobile undissolved particles, other than gas bubbles, unintentionally present in the solutions.” For small-volume injections — nominal content below 100 mL, which covers every vial discussed here — the light obscuration limits are not more than 6,000 particles per container at 10 µm and larger and not more than 600 at 25 µm and larger. Visual inspection against light and dark backgrounds is the practical complement: a vial showing particles, cloudiness or discolouration should not be used, whatever the certificate says.
Container closure integrity
The seal is part of the product. An elastomeric closure must reseal after each withdrawal and maintain the microbial barrier for the container's shelf life; coring, where the needle cuts out a fragment of stopper, both introduces a particle and damages the seal. Inspect the flip-top and crimp before first entry, and discard any vial whose closure integrity is in doubt. ↑ Back to top
How should bacteriostatic water be stored?
At controlled room temperature, 15–25 °C, protected from direct light, not frozen. Benzyl alcohol is a stable small molecule and the solution is not temperature-sensitive the way a lyophilised peptide is, so refrigeration offers no benefit and freezing risks the container and the closure seal. Keep the flip-top in place until first entry, and date the vial at first puncture so the in-use period is tracked against a written date rather than a memory. ↑ Back to top
What are the known limits of benzyl alcohol?
It is contraindicated in neonates
This is the best-documented limitation, and it is not theoretical. In June 1982 the US CDC's Morbidity and Mortality Weekly Report (31(22):290) described 16 neonatal deaths reported to the FDA from two medical centres — ten at one over six months, six at the other over sixteen. The infants were preterm neonates with central intravascular catheters flushed daily with bacteriostatic saline containing 9 mg/mL benzyl alcohol. The picture was metabolic acidosis progressing to respiratory distress, CNS signs including convulsions and intracranial haemorrhage, and late hypotension often preceding death; benzyl alcohol or its metabolites were detected in blood and urine at estimated intakes of 99 to 405 mg/kg/day. Both hospitals reported no further such deaths after switching to preservative-free flush solutions. The syndrome became known as gasping syndrome, and current labelling carries the warning directly: benzyl alcohol has been associated with toxicity in neonates, and preservative-free Sterile Water for Injection should be used for this population.
It can promote aggregation of some proteins
This is why a number of protein and peptide formulations are supplied preservative-free. Benzyl alcohol binds partially unfolded protein states and hydrophobic surfaces exposed during unfolding, shifting the conformational equilibrium and accelerating aggregation in susceptible sequences. Thirumangalathu and colleagues characterised this for recombinant human granulocyte colony-stimulating factor, mapping how pH, temperature and sucrose modulate benzyl alcohol-induced aggregation (J Pharm Sci 2006;95(7):1480–97). Bis and Mallela later reported that preservatives aggregate interferon alpha-2a in a rank order independent of the protein (Int J Pharm 2014;472(1–2):356–61) — susceptibility is a property of the sequence and its stability, not something readable off the preservative alone.
The practical consequence is that a preserved diluent is not automatically the right diluent for every peptide. Whether a sequence tolerates benzyl alcohol is an empirical question answered by the researcher's own stability work, not by a vendor page. We are not telling you which to use; we are telling you the variable exists and is sequence-dependent.
The label concentration is not always 0.9%
The Pfizer/Hospira labelling covers both 0.9% (9 mg/mL) and 1.1% (11 mg/mL) presentations. If the exact preservative concentration matters to an experiment — and in protein stability work it can — read it off the vial rather than assuming. ↑ Back to top
A note on arithmetic, and what this page will not tell you
Concentration is mass divided by volume. That is arithmetic, not guidance.
One factual correction belongs here, because it is routinely got wrong. When the mass term refers to a lyophilised research peptide, the correct figure is the measured content on that lot's certificate of analysis, not the number printed on the label. Label weight is a nominal declaration; measured content is an analytical result, and the two differ. Our current Epithalon lot is a concrete example: labelled 10 mg, measured at 11.75 mg by Janoshik Analytical under report #230585. A calculation using the label weight for that lot is wrong by 17.5% before anything else happens. Our guide to reading a peptide COA explains where to find the content figure and how it differs from purity.
Beyond that, this page stops. It states no volume, no concentration target, no “typical” figure and no worked example, because any of those would function as preparation guidance for a product sold as a laboratory reagent and not approved for human use. Nearly every competing page on this subject answers “how much water do I add.” We are not going to, and we would rather say so plainly than let the omission look like an oversight. ↑ Back to top
Bacteriostatic Water 3mL — $12.99 CAD → Bacteriostatic Water 10mL — $22.00 CAD →
Frequently asked questions
What is bacteriostatic water made of?
Water for Injection USP — purified, non-pyrogenic water — plus benzyl alcohol as a bacteriostatic preservative. The Pfizer/Hospira labelling gives the preservative content as 0.9% (9 mg/mL) or 1.1% (11 mg/mL) depending on presentation, with a pH of 5.7 and a specification range of 4.5 to 7.0. There is no buffer and no other additive.
What is the difference between bacteriostatic water and sterile water?
One ingredient, with large consequences. Sterile Water for Injection USP contains no bacteriostat, antimicrobial agent or added buffer and is supplied only in single-dose containers. Bacteriostatic Water contains benzyl alcohol and is supplied in a multiple-dose container from which repeated withdrawals may be made. The preservative is what permits more than one entry.
Why can a bacteriostatic water vial be punctured more than once?
Because benzyl alcohol suppresses multiplication of organisms a needle entry may introduce. USP General Chapter <51> requires this efficacy be demonstrated for all injections in multiple-dose containers, by challenging the sealed product with five organisms — Candida albicans, Aspergillus brasiliensis, Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus — and measuring log reductions at 14 and 28 days.
How long is bacteriostatic water good for after the first puncture?
The manufacturer's labelling governs. The common default, from USP General Chapter <797> as summarised by the CDC, is that an opened multi-dose vial is dated and discarded within 28 days of first entry unless the manufacturer specifies otherwise, and the assigned date can never exceed the original expiration date on the container. This is a limit on the vial, dated from first puncture — not a stability figure for anything prepared with it.
Does the preservative make bacteriostatic water sterile?
No. The water is sterile because it was sterilised during manufacture. Benzyl alcohol is bacteriostatic, meaning it inhibits bacterial multiplication rather than killing organisms. The CDC notes that a preservative in a multi-dose vial has no effect on viruses and does not provide complete protection against bacterial contamination. It is not a substitute for aseptic technique.
Why is benzyl alcohol contraindicated in newborns?
In 1982 the CDC's MMWR (31(22):290) reported 16 neonatal deaths associated with benzyl alcohol in intravascular flush solutions at two medical centres; the preterm infants received an estimated 99 to 405 mg/kg/day, and both hospitals reported no further such deaths after switching to preservative-free solutions. The syndrome is known as gasping syndrome. Current labelling states that preservative-free Sterile Water for Injection should be used for this population.
Can benzyl alcohol affect peptides or proteins?
It can, depending on the sequence. Benzyl alcohol binds partially unfolded protein states and hydrophobic surfaces exposed during unfolding, which can accelerate aggregation. Thirumangalathu and colleagues characterised this for recombinant human G-CSF (J Pharm Sci 2006;95(7):1480-97), and Bis and Mallela reported that preservatives aggregate interferon alpha-2a in an order independent of the protein (Int J Pharm 2014;472(1-2):356-61). This is why some protein formulations are prepared preservative-free. Whether a given sequence is affected is an empirical question for the researcher.
Does this page say how much water to add to a vial?
No. This page contains no reconstitution volume, no concentration target, no "typical" volume and no worked dilution example, and ThePeptide.ca does not provide preparation, dilution or administration guidance for any product it sells. The one arithmetic point worth stating is that concentration is mass divided by volume, and that the mass term for a lyophilised research peptide is the measured content on that lot's certificate of analysis, not the label weight.
Related resources
- Bacteriostatic Water 3mL · Bacteriostatic Water 10mL
- How to store research peptides: complete stability guide
- How to read a peptide COA · Where to get peptides tested in Canada · Are peptides legal in Canada?
References
- Bacteriostatic Water for Injection, USP — prescribing information (Hospira/Pfizer). DESCRIPTION and WARNINGS sections: benzyl alcohol 0.9% (9 mg/mL) or 1.1% (11 mg/mL); pH 5.7 (4.5–7.0); multiple-dose container; neonatal warning. labeling.pfizer.com
- Sterile Water for Injection, USP — prescribing information (Hospira/Pfizer). “Contains no bacteriostat, antimicrobial agent or added buffer”; “supplied only in single-dose containers”; haemolysis warning. labeling.pfizer.com
- United States Pharmacopeia. General Chapter <51> Antimicrobial Effectiveness Testing. Challenge organisms, Category 1 criteria, log reductions at 14 and 28 days. Chapter text
- United States Pharmacopeia. General Chapter <71> Sterility Tests. Media, 14-day incubation, and the chapter's own statement of its limits. Chapter text
- United States Pharmacopeia. General Chapter <788> Particulate Matter in Injections. Small-volume limits: ≤6,000 particles/container ≥10 µm; ≤600/container ≥25 µm by light obscuration. Chapter text
- US Centers for Disease Control and Prevention. Preventing Unsafe Injection Practices (clinical safety). Multi-dose vials, the 28-day USP <797> limit, and what preservatives do and do not do. cdc.gov
- Centers for Disease Control and Prevention. Neonatal Deaths Associated With Use Of Benzyl Alcohol — United States. MMWR. 1982;31(22):290. cdc.gov/mmwr
- Thirumangalathu R, Krishnan S, Brems DN, Randolph TW, Carpenter JF. Effects of pH, temperature, and sucrose on benzyl alcohol-induced aggregation of recombinant human granulocyte colony stimulating factor. J Pharm Sci. 2006;95(7):1480–1497. PMID 16729274. PubMed
- Bis RL, Mallela KMG. Antimicrobial preservatives induce aggregation of interferon alpha-2a: the order in which preservatives induce protein aggregation is independent of the protein. Int J Pharm. 2014;472(1–2):356–361. PMID 24974985. PubMed