Bacteriostatic water: what it is and how long it lasts
Bacteriostatic water is sterile water containing 0.9% benzyl alcohol as a preservative, which is what allows the same vial to be punctured repeatedly. Manufacturer labelling commonly specifies discarding it 28 days after the first puncture. Sterile water for injection contains no preservative and is intended for a single use.
What it is
The benzyl alcohol does one job: it inhibits bacterial growth in a vial that will be entered more than once. That is the entire difference, and it is the reason multi-dose reconstitution uses it.
Versus sterile water
| Bacteriostatic water | Sterile water for injection | |
|---|---|---|
| Preservative | 0.9% benzyl alcohol | none |
| Intended use | multi-dose | single dose |
| After first puncture | commonly labelled 28 days | discard |
Which diluent is appropriate for a given product is set by that product's labelling and your prescriber, not by convenience.
How much to use
The volume does not change the dose, only how the dose reads on your syringe. More water means a lower concentration and a larger, easier-to-read unit number for the same milligrams. Pick a volume that puts your usual dose comfortably on the barrel and then stay consistent across vials, because switching volumes between vials is how people misdose. Work out your number →
Storage and expiry
Store as the label directs, keep it out of light and heat, label the vial with the date of first puncture, and discard by the labelled date rather than by how it looks. Benzyl alcohol is also a reason certain populations are advised against products containing it, which is a prescriber conversation.
Track the vial, not just the dose
Bo logs when each vial was opened and what concentration it holds, so the unit number on your reminder always matches the vial you are actually using.
Educational information only. Bo is a tracking app, not a medical device, and nothing here is medical advice, a dosing recommendation, or a substitute for your prescriber. Half-lives are population averages from the sources cited on each row; individual clearance varies with body weight, kidney and liver function, and other medication.