Bacteriostatic Water Explained: What It Is and When to Use It
Bacteriostatic water is the most used — and most misunderstood — consumable in peptide research. This short guide covers what it actually is, how it differs from sterile water, and when each belongs in a protocol.
What it is
Bacteriostatic water (often "BAC water") is sterile water containing 0.9% benzyl alcohol as a preservative. The name 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. The benzyl alcohol concentration is low enough to leave peptide chemistry alone at research scales 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 — which is why 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 is a backstop, not a license for sloppy technique.
- Discard at 30 days, or immediately on any cloudiness or particulates.
- Note that 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.
We stock USP-grade bacteriostatic water in 10mL multi-dose vials, tested and certified like every product in the catalogue. For research use only — not for human or veterinary use. Next: how to reconstitute peptides, step by step.