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Research use only
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bacteriostatic water is sterile water for injection containing 0.9% (9 mg/mL) benzyl alcohol as a preservative. That single added component is what lets one reconstituted reagent vial be re-entered repeatedly over a working period without supporting microbial growth, which is why it is a common solvent for bench reconstitution of lyophilised research peptides. This guide explains the composition, how it differs from plain sterile water and saline, and the laboratory handling conventions that go with a multiple-withdrawal vial. It is not a preparation for any use in humans or animals.
| Common name | Bacteriostatic Water for Injection, USP (the pharmacopoeial product name) |
|---|---|
| Composition | Water for injection plus 0.9% (9 mg/mL) benzyl alcohol |
| Preservative | Benzyl alcohol, a bacteriostatic agent (inhibits growth rather than killing all organisms) |
| Stated pH | 5.7 (range 4.5 to 7.0) |
| Container type | Multiple-dose container; repeated withdrawals from one vial are possible |
| Re-entry reference | Clinical injection-safety guidance dates a punctured multiple-dose vial and discards it within 28 days (CDC) |
| Research role | Solvent for reconstituting lyophilised research peptides from a re-entered reagent vial, in the laboratory |
Bacteriostatic Water for Injection, USP is described by the pharmacopoeial and manufacturer labelling as a sterile, nonpyrogenic preparation of water for injection containing 0.9% (9 mg/mL) of benzyl alcohol added as a bacteriostatic preservative. Some presentations use a 1.1% (11 mg/mL) benzyl alcohol concentration, but the 0.9% formulation is the grade referenced throughout this guide. The stated pH is 5.7, within a range of 4.5 to 7.0, and the product is supplied in a multiple-dose container from which repeated withdrawals may be made. That last property, the ability to re-enter one vial repeatedly, is what makes it a convenient solvent for bench reconstitution of dry reagents. Kovalabs supplies bacteriostatic water strictly as a research reagent; it is not a preparation for any use in humans or animals. You can find the product page at /products/bacteriostatic-water. This guide explains the chemistry, while the laboratory procedure for using it lives separately on the reconstitution guide.
Benzyl alcohol is the single component that distinguishes bacteriostatic water from plain sterile water. It is a bacteriostatic agent, which means it inhibits the multiplication of bacteria rather than necessarily killing every organism present. The distinction matters: bacteriostatic is not the same as bactericidal. Because the preservative suppresses microbial growth between withdrawals, a single reconstituted vial can be re-entered repeatedly across a working period without becoming a growth medium, where a preservative-free solvent could not. Benzyl alcohol is a widely used aqueous-soluble antimicrobial preservative in injectable preparations, and is generally understood to act by disrupting microbial cell-membrane integrity. The label-verified point is that the 0.9% benzyl alcohol functions as the bacteriostatic preservative.
Three aqueous solvents are commonly discussed together, and they line up cleanly by what is added to the water. Bacteriostatic water is water for injection plus 0.9% (9 mg/mL) benzyl alcohol, which is what makes it suitable for repeated withdrawal from one vial. Sterile Water for Injection, USP is a sterile, nonpyrogenic preparation of water for injection that contains no bacteriostat, antimicrobial agent or added buffer, and is supplied only in single-dose containers; lacking a preservative, it is not suited to repeated re-entry of one vial over time. 0.9% Sodium Chloride Injection, USP is a sterile, nonpyrogenic, isotonic solution in which each mL contains 9 mg of sodium chloride, again with no bacteriostat or added buffer, and in its preservative-free presentation is for single use. It adds dissolved salt to the solvent rather than a preservative. The composition and single- or multiple-dose status of all three come directly from the respective pharmacopoeial and manufacturer labels. The table above sets them side by side.
| Solvent | Composition | Preservative | Container type | Typical research role |
|---|---|---|---|---|
| Bacteriostatic water for injection | Water for injection + 0.9% (9 mg/mL) benzyl alcohol | Yes, benzyl alcohol | Multiple-dose, re-entry possible | Repeated reconstitution from one re-entered reagent vial over a working period |
| Sterile water for injection | Water for injection only | None | Single-dose only | Where a single-use, preservative-free solvent is wanted |
| 0.9% sodium chloride | Water + 9 mg/mL sodium chloride (isotonic) | None | Single-dose (preservative-free presentation) | Where an isotonic, preservative-free diluent is wanted |
Lyophilised (freeze-dried) research peptides are supplied as a dry cake and must be returned to solution with an aqueous solvent before analysis or aliquoting in the laboratory. Bacteriostatic water is chosen for bench reconstitution specifically when a single reconstituted vial will be re-entered multiple times over a working period, because the benzyl alcohol suppresses microbial growth between withdrawals. Where a single-use, preservative-free, or isotonic solvent is preferred instead, sterile water or saline are the alternatives. This guide stops at the choice of solvent. The step-by-step reconstitution procedure, including the calculator, lives on the reconstitution guide and is not repeated here. Reconstitution in this context is the preparation of a reagent solution for analysis, not a preparation for any use in humans or animals.
Clinical injection-safety guidance, as a handling reference, dates a multiple-dose vial once it is opened or needle-punctured and discards it within 28 days unless the manufacturer states another date (CDC). The preservative is not a substitute for clean technique: the CDC notes that benzyl alcohol does not act on viruses and does not provide complete protection against bacterial contamination, so aseptic handling of the reagent vial still matters. For storage, general handling guidance across commercial and laboratory sources is to keep the vial upright at controlled room temperature, commonly cited as 15 to 30 C (59 to 86 F), away from direct light, and to mark the date of first puncture so the working window can be tracked. Those specific figures are general handling guidance rather than a quoted pharmacopoeial limit. The USP definition of controlled room temperature is 20 to 25 C, with excursions permitted between 15 and 30 C, if you want a pharmacopoeial anchor. Unopened shelf life is frequently described as up to about two years, but this is product- and lot-specific. Follow the expiry printed on the individual vial rather than any universal figure.
It is sterile, nonpyrogenic water for injection with 0.9% (9 mg/mL) benzyl alcohol added as a bacteriostatic preservative. The stated pH is 5.7, within a range of 4.5 to 7.0. Some presentations use 1.1% (11 mg/mL) benzyl alcohol, but 0.9% is the grade referenced here for laboratory reconstitution.
Benzyl alcohol. Sterile water for injection contains no preservative and is supplied only in single-dose containers, so it is not suited to repeated re-entry of one vial. Bacteriostatic water contains 0.9% benzyl alcohol and is supplied in a multiple-dose container from which repeated withdrawals may be made.
It is a bacteriostatic agent, meaning it inhibits the multiplication of bacteria rather than necessarily killing all organisms. That suppression of microbial growth between withdrawals is what allows one reconstituted reagent vial to be re-entered repeatedly over a working period without supporting growth.
Clinical injection-safety guidance, used here as a handling reference, dates a punctured multiple-dose vial and discards it within 28 days unless the manufacturer states another date (CDC). Aseptic handling still matters, because the preservative does not act on viruses and does not fully protect against bacterial contamination.
General handling guidance is to keep the vial upright at controlled room temperature, commonly cited as 15 to 30 C (59 to 86 F), away from direct light, and to mark the date of first puncture. The USP controlled room temperature definition is 20 to 25 C with excursions permitted between 15 and 30 C. For unopened shelf life, follow the expiry printed on the vial rather than a universal figure.
In a laboratory setting, bacteriostatic water is a common solvent for returning lyophilised research peptides to solution when a single reconstituted reagent vial will be re-entered multiple times for analysis. For the procedure, see the reconstitution guide. All Kovalabs products are supplied strictly for research use only, and are not a preparation for any use in humans or animals.