The short definition
Bacteriostatic water (often shortened to “BAC water”) is sterile water that has 0.9% benzyl alcohol added as a preservative. The word bacteriostatic means “stops bacteria from multiplying” — and that is exactly what the benzyl alcohol does. It does not sterilize the water by killing every organism on contact; instead it holds bacterial growth in check so the vial stays usable across repeated entries.
That single property — repeated-entry without loss of sterility — is the entire reason bacteriostatic water exists as a distinct product from plain sterile water.
What is in it: 0.9% benzyl alcohol
The formulation is deliberately simple:
- Water for injection — purified, sterile-filtered water that meets pharmacopeial standards for particulates and endotoxins.
- 0.9% benzyl alcohol (9 mg per mL) — the bacteriostatic agent. Benzyl alcohol is a well-characterised preservative that suppresses microbial growth at this concentration.
There is no salt, no buffer, and no active ingredient. The pH is roughly neutral. The only job of the product is to be a clean, preserved diluent.
Why the benzyl alcohol matters
A standard single-use vial of sterile water is sterile only until the moment a needle breaks the seal. After that first entry, any organism introduced can multiply freely in the nutrient-free but warm, wet environment. That is why sterile water is labelled single-dose and discarded after one use.
Benzyl alcohol changes the math. Because it continuously inhibits bacterial growth, a multi-dose bacteriostatic water vial can be entered repeatedly. In practice, laboratories treat an opened vial as usable for up to 28 days when stored and handled per their protocol — the same window cited in USP labelling for multi-dose preserved water.
How it is used in research
In a research setting, bacteriostatic water is used as a reconstitution and dilution diluent — the clean liquid that lyophilized (freeze-dried) research materials are dissolved into before laboratory work. The multi-dose format is convenient when a single vial of material is drawn down over several sessions, because the diluent vial can be re-entered without being thrown away each time.
This guide does not describe preparation of anything for human or animal administration. Bacteriostatic water sold here is supplied strictly for laboratory and research applications.
How to recognise a quality product
Not all bacteriostatic water is documented to the same standard. When evaluating a supplier, look for:
- Sterile-filtered, USP-grade water with the 0.9% benzyl alcohol concentration stated on the label.
- Lot-level documentation — a traceable lot number so a specific vial maps to a specific production run.
- Sensible packaging — a sealed multi-dose vial with an intact stopper, stored away from light.
- A real support channel in case you have a question about a lot.
Our 10 mL multi-dose vials are sterile-filtered, made with 0.9% benzyl alcohol, and documented down to the lot. See the product.
Frequently asked questions
What is bacteriostatic water made of?
Sterile water for injection plus 0.9% benzyl alcohol (9 mg/mL) as a bacteriostatic preservative. There is no salt, buffer, or active ingredient.
What does bacteriostatic mean?
Bacteriostatic means it stops bacteria from multiplying, rather than killing them outright (bactericidal). The benzyl alcohol holds microbial growth in check so a multi-dose vial stays usable across repeated entries.
Is bacteriostatic water the same as sterile water?
No. Both start sterile, but only bacteriostatic water contains a preservative. Plain sterile water is single-use; bacteriostatic water is designed for repeated entry over roughly 28 days.