Short answer. Bacteriostatic (BAC) water is sterile water with a preservative so a multi-dose bottle can be used more than once. Keep it in the fridge, capped. Once opened, write the first-use date on the bottle and plan to discard it by about 28 days, or sooner if the label says so. This page is storage only — it does not tell you how much to add to a vial.
People usually ask three things: what BAC water is, where the bottle lives, and how long an opened bottle stays in play. Those are the only questions this page answers.
Bacteriostatic water is sterile water that includes a small amount of preservative — typically benzyl alcohol. The preservative is there so the stopper can be pierced more than once under clean technique. It is not the same as plain sterile water, which has no preservative and is usually treated as single-use.
This guide does not cover reconstitution volumes, syringe fills, or protocols. For vial storage after mixing, see the peptide storage guide.
| State | Usual practice |
|---|---|
| Sealed, unopened | Follow the expiry printed on the bottle; store refrigerated. |
| Opened (first puncture) | Commonly discarded by about 28 days from first use, refrigerated the whole time — or earlier if the label says so. |
| Discard sooner if | Cloudiness, particles, a damaged stopper, a broken seal, or any other change from how it looked when new. |
The 28-day figure is a widely used beyond-use habit for preserved multi-dose diluents once the stopper has been entered. Your bottle’s own label always wins if it gives a shorter window.
No mixing volumes, no “how many ml to add,” no dose maths, and no protocols. For unit conversion only, use syringe units to ml or mg to mcg.