Bacteriostatic Water Explained: What It Is and When to Use It
Bacteriostatic water is widely used in peptide research, but the term is often misunderstood. Its composition and preservative function determine when each diluent fits a protocol.
What it is
Bacteriostatic water (often "BAC water") is sterile water containing 0.9% benzyl alcohol as a preservative. The term is precise: bacteriostatic means it stops bacteria from multiplying. It does not kill organisms already present (that would be bactericidal), and it is not a sterilant. At research scales, the benzyl alcohol concentration is low enough not to alter peptide chemistry while suppressing microbial growth for the labeled use period.
Why multi-dose vials need it
Every needle puncture of a vial septum is a potential contamination event. In plain sterile water, a single introduced organism can multiply freely. For that reason, unpreserved vials are single-use. With 0.9% benzyl alcohol present, incidental contaminants are held static, letting one 10mL vial serve a multi-week protocol with repeated draws. The accepted working window is 28–30 days from first puncture, refrigerated.
Bacteriostatic vs sterile water at a glance
| Bacteriostatic water | Sterile water | |
|---|---|---|
| Preservative | 0.9% benzyl alcohol | None |
| Multi-dose use | Yes, ~30 days refrigerated | No; single session |
| Typical use | Ongoing research protocols | Same-day, single-draw work |
| Peptide compatibility | Standard for research peptides | Also compatible; shorter window |
Handling practice
- Refrigerate after first puncture and date the vial.
- Swab the septum before every draw. The preservative supplements aseptic technique; it does not replace it.
- Discard at 30 days, or immediately on any cloudiness or particulates.
- A small number of sensitive biological assays exclude benzyl alcohol; if your downstream assay is one of them, use sterile water and single-session handling instead.
For research use only — not for human or veterinary use. For the full procedure, see how to reconstitute peptides, step by step.