Bacteriostatic water is sterile water that contains a small amount of benzyl alcohol — about 0.9% (9 mg/mL) — added as a bacteriostatic, meaning growth-inhibiting, preservative1. In plain terms: it is water with a mild germ-suppressant built in, so a vial can be punctured and drawn from again and again without turning into a breeding ground for bacteria. That preservative is the whole reason it exists.
Bacteriostatic vs. the alternatives
| Diluent | Preservative | Typical use |
|---|---|---|
| Bacteriostatic water | Benzyl alcohol ~0.9% | Multi-use reconstitution over days/weeks |
| Sterile water for injection | None | Single-use only |
| Sterile saline (0.9% NaCl) | None | Single-use; isotonic |
Because reconstituted peptide vials are usually entered many times across a protocol, bacteriostatic water is the standard choice: the preservative keeps a repeatedly-punctured vial usable1.
Why benzyl alcohol matters
The benzyl alcohol doesn't sterilise the peptide or repair it — it simply suppresses microbial growth between uses. In plain terms: it protects the water from bacteria, not the drug from breaking down. This is why a bacteriostatic-water-reconstituted vial has a meaningfully longer usable window than one mixed with plain sterile water, and why refrigeration plus that preservative together define a peptide's shelf life once mixed.
Benzyl alcohol is a well-established pharmaceutical preservative used in small amounts, but it is not for everyone: the labelled product is contraindicated in newborns, where benzyl alcohol has been linked to serious toxicity1. This is general reference information, not medical guidance.
Practical points
- Store bacteriostatic water per its label; it too has an expiry, and its preservative window is finite once opened.
- The volume you draw for reconstitution is a design choice — see how to reconstitute a peptide.
- Once mixed, refrigerate and track the date — how long reconstituted peptides last.