Bacteriostatic Water: What It Is and Why It Matters

Bacteriostatic water is one of the most commonly used — and most commonly misunderstood — items in a peptide research setting. It looks identical to ordinary water, but a small additive makes an important difference to how it behaves once a vial is opened.

What It Actually Is

Bacteriostatic water is water for injection that contains 0.9% benzyl alcohol as a preservative. The benzyl alcohol is bacteriostatic — meaning it inhibits the growth of bacteria — which is what allows a vial to be entered multiple times over a period of days without the contents becoming a breeding ground for microbes.

Bacteriostatic vs. Sterile Water

Plain sterile water contains no preservative. It's sterile at the moment it's sealed, but once the vial is opened and a needle is introduced, there's nothing to inhibit microbial growth — so sterile water is generally intended for single use. Bacteriostatic water, by contrast, is suited to multi-use vials because the preservative keeps it usable across repeated draws. For most laboratory peptide reconstitution, where a single vial is drawn from several times, bacteriostatic water is the standard choice.

Why It's Used for Peptides

Lyophilised peptides are supplied as a dry powder and must be dissolved before use. Bacteriostatic water is the usual diluent because it's chemically compatible with most peptides at standard laboratory dilutions and, crucially, keeps a reconstituted multi-use vial from becoming contaminated between draws. The benzyl alcohol inhibits microbial growth but does not stop the chemical degradation of the peptide itself — which is why reconstituted vials still need refrigeration and prompt use.

Handling and Storage

Store bacteriostatic water at room temperature, away from direct light and heat. Sterilise the stopper with an alcohol swab before each entry, and follow your laboratory's standard protocol for how long an opened vial remains suitable for use. For the full reconstitution method, see our guide on reconstituting lyophilised peptides.