Technical Reference
Bacteriostatic Water
Bacteriostatic water is sterile, non-pyrogenic water that contains 0.9% benzyl alcohol (9 mg/mL) as an added bacteriostatic preservative. The benzyl alcohol inhibits the multiplication of common bacterial contaminants, which is what allows a single multi-dose vial to be entered repeatedly — conventionally for up to 28 days after the first puncture — rather than being discarded after one use like plain sterile water. It is supplied as a clear, colorless solution, most commonly in 10 mL, 20 mL, and 30 mL multi-dose vials; 10 mL is the standard research-supply unit.
For research and laboratory use only · Not for human or veterinary use
Specification at a glance
- Composition: sterile, non-pyrogenic water with 0.9% benzyl alcohol (9 mg/mL) as preservative
- Bacteriostatic (inhibits bacterial multiplication) — not bactericidal (does not kill bacteria)
- Multi-dose: conventionally usable up to 28 days after the first vial puncture
- Compendial tests: USP <71> sterility, USP <85> bacterial endotoxins, USP <788> particulates
- Common formats: 10 mL, 20 mL, and 30 mL multi-dose vials (BAC Water Depot supplies 10 mL, Type I borosilicate glass)
- Appearance specification: clear, colorless solution free of visible particulate matter
- Unopened storage: controlled room temperature 20–25 °C (68–77 °F); excursions permitted 15–30 °C. After first puncture, follow the vial label
- Distinct from sterile water (no preservative, single-use) and from distilled/deionized water (purity grades, not sterile specifications)
How bacteriostatic water compares to other waters
| Type | Preservative | Sterile | Reuse after first entry |
|---|---|---|---|
| Bacteriostatic water | 0.9% benzyl alcohol | Yes | Multi-dose — up to 28 days after first entry |
| Sterile water for injection | None | Yes | Single-use — discard after one entry |
| Bacteriostatic saline | Benzyl alcohol | Yes | Multi-dose — also contains 0.9% sodium chloride |
| Distilled water | None | Not inherently | Purity grade, not a sterile diluent specification |
| Deionized water | None | Not inherently | Purity grade, not a sterile diluent specification |
Deeper comparisons: bacteriostatic vs sterile water, vs deionized water, vs distilled water.
Frequently asked questions
What is bacteriostatic water?
Bacteriostatic water is sterile, non-pyrogenic water that contains 0.9% benzyl alcohol (9 mg/mL) as an added bacteriostatic preservative. The benzyl alcohol inhibits the multiplication of common bacterial contaminants, which is what allows a single multi-dose vial to be entered repeatedly — conventionally for up to 28 days after the first puncture — rather than being discarded after one use like plain sterile water. It is supplied as a clear, colorless solution, most commonly in 10 mL, 20 mL, and 30 mL multi-dose vials; 10 mL is the standard research-supply unit.
What is bacteriostatic water made of?
Two components: water and a preservative. The base is sterile, non-pyrogenic water purified to injection-grade specification, and the preservative is 0.9% benzyl alcohol — 9 mg per mL. Nothing else is added; there is no buffer, salt, or tonicity agent, which is what distinguishes it from bacteriostatic saline (which also contains 0.9% sodium chloride).
How does the benzyl alcohol work?
Benzyl alcohol is bacteriostatic, not bactericidal — an important distinction. Bacteriostatic means it inhibits bacteria from multiplying; bactericidal means it actively kills them. Benzyl alcohol disrupts bacterial membrane permeability enough to suppress reproduction, so organisms introduced during a needle entry do not proliferate to significant levels before the vial's in-use window ends.
What is bacteriostatic water used for?
In laboratory and research settings it is the standard diluent for reconstituting lyophilized (freeze-dried) compounds into solution. Its defining advantage is multi-dose capability: because the preservative suppresses bacterial growth, one vial can be drawn from across multiple withdrawals over days or weeks, whereas an unpreserved diluent must be discarded after a single entry.
What is the difference between bacteriostatic water and sterile water?
Preservative and reusability. Sterile water for injection contains no preservative, so once the vial is entered it is single-use and any remainder is discarded. Bacteriostatic water contains 0.9% benzyl alcohol, so the same vial can be entered repeatedly for up to 28 days. Both start sterile; only bacteriostatic water is designed to stay usable after the first puncture.
How is bacteriostatic water different from distilled and deionized water?
Distilled and deionized water are purification grades, not sterility grades. Both describe how dissolved minerals or ions were removed, and neither is inherently sterile, non-pyrogenic, or preserved. Bacteriostatic water is a finished sterile specification: purified, sterile-filtered, endotoxin-controlled, and preserved. Purity grade and sterility are separate properties, and only sterility matters for a sterile diluent.
What standards is bacteriostatic water tested against?
The recognized compendial tests are USP <71> for sterility, USP <85> for bacterial endotoxins, USP <788> for particulate matter, and a chromatographic assay (commonly HPLC) to confirm the benzyl alcohol concentration. A per-lot Certificate of Analysis reports the measured result for each test against its specification for that specific production lot.
How should bacteriostatic water be stored?
Unopened, store at controlled room temperature — USP defines this as 20–25 °C (68–77 °F), with excursions permitted between 15–30 °C (59–86 °F) — away from direct light, per the vial label. The in-use window begins at first puncture, not when the carton is opened, and is conventionally 28 days. Sealed vials carry a manufacturer expiration date that is independent of the in-use window.
How long does bacteriostatic water last after opening?
The conventional multi-dose in-use period is 28 days from first puncture. That limit reflects the preservative's capacity to suppress growth through repeated entries, not the sterility of the unopened vial. An unpunctured vial remains usable until its labeled expiration date, typically 18–24 months from manufacture.
Why is bacteriostatic water sold for research use only?
Because research-supply bacteriostatic water is not an FDA-approved drug product and is not dispensed by a pharmacy or prescriber. That is a regulatory distinction, not a statement about the substance. BAC Water Depot supplies it strictly as a laboratory diluent for research use — not for human or veterinary administration — and does not publish clinical dosing guidance. The FDA-approved clinical product carries its own human-use labeling, which does not apply to research-grade material.
Detailed references
- What is bacteriostatic water — full guide
- Benzyl alcohol: the preservative explained
- Shelf life and the 28-day in-use window
- FDA classification and regulatory status
- Storage requirements
- Storage after opening
- Certificate of Analysis — what a lot-specific CoA reports
- How to identify genuine bacteriostatic water
BAC Water Depot supplies research-grade bacteriostatic water in 10 mL multi-dose vials, manufactured in the USA in an ISO 9001:2015 registered facility, with per-lot testing by an independent third-party laboratory and a lot-specific Certificate of Analysis reachable by scanning the QR code on the vial. Single vials $9.99, 10-pack $74.99, bulk from $6.49 per vial.
