Bacteriostatic water is essentially water for injection with a small amount of preservative added. It sounds like a trivial difference, but that difference is exactly what determines how long a vial can be used after the first puncture. Here is what the preservative is and why it matters.
What bacteriostatic water is, briefly
It is sterile water with added benzyl alcohol (usually around 0.9%) as a preservative. The word "bacteriostatic" means not "sterilizes" but "holds back bacterial growth": the preservative does not kill everything instantly, it prevents microorganisms from multiplying in a multi-use vial. More detail is in the bacteriostatic water monograph.
Why the preservative is there
Plain water for injection contains no preservative, so it is meant for single use: once opened, it is no longer protected against contamination. The role and purpose of antimicrobial preservatives in parenteral products is reviewed in detail in the literature (J. Pharm. Sci., 2007). Adding benzyl alcohol is what lets a vial be punctured many times over a limited period without bacterial growth between draws.
On benzyl alcohol safety
An honest note is needed here. Benzyl alcohol is not a neutral substance at any amount: the so-called gasping syndrome, a severe toxic reaction in newborns linked to benzyl alcohol, is a documented historical case (N. Engl. J. Med., 1982). Products with this preservative are therefore flagged as unsuitable for certain groups. That is another reason such a diluent is a topic for a specialist, not for arbitrary self-administration.
Research status
Longeva material is research use only. It is a consumable for laboratory work, not a medicine and not an instruction for medical procedures. Technical usage details are in a separate document, not in this overview.
Lots come with a certificate of analysis from an independent third-party laboratory; a COA (certificate of analysis) by lot number is in the open archive. For presentation, see the bacteriostatic water catalogue page.
