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Research use only
· 5 min read
The name sounds like a warning label, but the chemistry is straightforward. Bacteriostatic water is water for injection with exactly one additive: 0.9% (9 mg/mL) benzyl alcohol. That single component is why a reconstituted reagent vial can be re-entered repeatedly over a working period without becoming a culture medium. Strip it out and what remains is sterile water for injection, which is not the same thing and is not suited to repeated withdrawal from one vial. This guide covers the composition, how it sits relative to sterile water and saline, and the laboratory handling conventions that go with a multiple-dose container. It is supplied strictly as a research reagent and 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 in pharmacopoeial and manufacturer labelling as a sterile, nonpyrogenic preparation of water for injection containing 0.9% (9 mg/mL) benzyl alcohol as a bacteriostatic preservative. Some presentations use a 1.1% (11 mg/mL) concentration, but 0.9% is the grade referenced throughout this guide. The stated pH is 5.7, within a measured range of 4.5 to 7.0. It is supplied in a multiple-dose container - meaning repeated withdrawals from the same vial are explicitly part of the intended use profile. The Pfizer Medical product description and the DailyMed label records for both bacteriostatic and sterile water are the primary documentary sources; the reconstitution guide is where the laboratory procedure lives.
Benzyl alcohol is the single component that distinguishes bacteriostatic water from plain sterile water, and it is worth being precise about what bacteriostatic actually means. The preservative inhibits the multiplication of bacteria rather than necessarily killing every organism present. Bacteriostatic is not bactericidal, and that distinction is not pedantry: a bacteriostatic solvent suppresses growth between withdrawals, which is what makes re-entry of one vial across a working period defensible. A preservative-free solvent offers no such window.
Benzyl alcohol is a widely used aqueous-soluble antimicrobial preservative in parenteral-grade preparations and is generally understood to act by disrupting microbial cell-membrane integrity. The CDC notes explicitly that benzyl alcohol does not act on viruses and does not provide complete protection against bacterial contamination - which means aseptic handling of the reagent vial still matters even with the preservative present. The preservative buys time; it does not replace technique.
These three solvents get conflated in catalogue descriptions more often than they should. The differences are entirely compositional, and the table below makes them unambiguous. Sterile Water for Injection, USP contains no bacteriostat, no antimicrobial agent and no buffer, and is supplied only in single-dose containers - the absence of a preservative makes re-entry of one vial unsuitable. 0.9% Sodium Chloride Injection, USP adds dissolved salt (9 mg/mL NaCl) rather than a preservative, and in its preservative-free presentation is also single-use. The composition and container type of all three come directly from the respective pharmacopoeial and manufacturer labels.
| 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 research peptides are shipped as a dry cake and must be returned to solution with an aqueous solvent before analysis or aliquoting in the laboratory. Bacteriostatic water is selected for bench reconstitution specifically when a single reconstituted reagent vial will be re-entered multiple times across a working period, because the benzyl alcohol suppresses microbial growth between withdrawals. Where a single-use, preservative-free or isotonic solvent is preferred, sterile water or saline are the alternatives.
This guide stops at the choice of solvent. The step-by-step reconstitution procedure, including the concentration calculator, lives on the reconstitution guide and is not repeated here. Reconstitution in this context means preparation of a reagent solution for analysis, not a preparation method for any use in humans or animals.
Clinical injection-safety guidance - used here as a handling reference for vial management practice - dates a multiple-dose vial once it is first punctured and discards it within 28 days unless the manufacturer specifies a different date (CDC). That 28-day figure is not a pharmacopoeial shelf-life statement; it is an aseptic-practice recommendation for managing a re-entered container.
On storage, general 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 figures are 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. Unopened shelf life is frequently described as up to approximately two years, but this is product- and lot-specific. The expiry printed on the individual vial is the only reliable reference; no universal figure applies.
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.