Bacteriostatic Water vs. Sterile Water
Written and reviewed by Evan Marlow, developer of DosePlot · Updated 2026-08-27
The one difference that matters is a preservative. Bacteriostatic water contains 0.9% benzyl alcohol, which stops bacteria from growing so a vial can be entered repeatedly over days or weeks; sterile water has no preservative and is meant for a single use.[1][2] That single fact decides which one fits a multi-dose peptide vial.
Side by side
| Bacteriostatic water | Sterile water | |
|---|---|---|
| Preservative | 0.9% benzyl alcohol — bacteriostatic | None — no antimicrobial agent at all |
| Container type | Multiple-dose (repeated withdrawals) | Single-dose (discard the rest) |
| How long once opened | Date it; up to 28 days as a multi-dose vial | Use once; unused portion discarded |
| Labeled purpose | Diluent / solvent for injectable drugs | Diluent / solvent; must be made isotonic for IV |
| Best fit for | A vial you draw from many times over days/weeks | A single admixture used immediately |
Attributes drawn from each product's FDA-approved label.[1][2] The 28-day multi-dose figure is CDC injection-safety guidance.[3]
Why the preservative is the deciding factor
A peptide vial is typically drawn from many times. Without a preservative, every needle entry is a fresh chance to introduce bacteria into a solution that then sits in the fridge — and sterile water, which has no preservative, is labeled single-dose, with the unused portion discarded.[2]Bacteriostatic water's benzyl alcohol is what makes repeated withdrawals reasonable, and it is why a mixed multi-dose vial still gets dated and discarded within about a month rather than kept indefinitely.[3]
Two things worth knowing about benzyl alcohol
- Not for neonates. The bacteriostatic-water label carries an explicit warning against use in newborns — benzyl alcohol has been linked to serious toxicity in neonates.[1]
- It's a diluent, not a drug. Both waters are labeled for use only as a solvent or diluent for medications — never injected on their own. Sterile water in particular can cause hemolysis if given intravenously without being made isotonic first.[2]
Sources
- Bacteriostatic Water for Injection, USP — Hospira, Inc. FDA-approved prescribing information (via DailyMed): sterile, nonpyrogenic, 0.9% (9 mg/mL) benzyl alcohol as a bacteriostatic preservative; multiple-dose; diluent/solvent only; warning: not for use in neonates.
- Sterile Water for Injection, USP — FDA-approved label (via DailyMed): contains no bacteriostat, antimicrobial agent, or added buffer; single-dose only; intravenous administration without a solute may cause hemolysis, so it must be made approximately isotonic before IV use.
- CDC — Injection Safety: a multi-dose vial is dated when first accessed and discarded within 28 days unless the manufacturer specifies otherwise.
More in this guide
Part of the complete guide to reconstituting peptides.
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