Bacteriostatic vs. sterile water

When you reconstitute a peptide, the diluent you choose matters more than most people expect. The two options are bacteriostatic water and plain sterile water, and the difference is one ingredient: a preservative.

Bacteriostatic water

Bacteriostatic water is sterile water with 0.9% benzyl alcohol added. Benzyl alcohol is a preservative that suppresses bacterial growth (that is what “bacteriostatic” means). Because of it, a single vial can be entered with a needle repeatedly over weeks without the solution spoiling — which is exactly what a multi-dose peptide vial needs. It is the default diluent for the research peptides in this library.

Sterile water (SWFI)

Sterile water for injection is plain — no preservative. It is intended for single use: once a vial is entered there is nothing to hold back contamination, so it is not suited to a vial you will draw from over many days.

Which to use

• Drawing from one vial over days or weeks → bacteriostatic water. This is the usual case.
• Using an entire vial in a single sitting, or a known benzyl-alcohol sensitivity → sterile water can make sense.
• Never use tap, spring, distilled, or any non-injectable water — only pharmaceutical-grade water made for injection.

Inspect every vial before use and discard any solution that has turned cloudy or developed particulates. See BAC Water for the diluent's own page.