Both are water. The difference is a preservative, and that single difference decides how long a reconstituted vial remains usable and whether it belongs in your assay at all.
The short answer
Sterile Water for Injection is purified water that has been sterilised and contains nothing else. Once you open it or introduce anything into it, nothing stops microbial growth.
Bacteriostatic Water for Injection is the same sterile water with benzyl alcohol added as a preservative — 0.9% (9 mg/mL) in the common US presentation, though 1.1% (11 mg/mL) presentations also exist. Check the specific product label rather than assuming.
Benzyl alcohol is bacteriostatic, not bactericidal: it inhibits bacterial multiplication rather than sterilising on contact. That is enough to make a multi-draw vial practical over a period of days.
Side by side
| Composition | Sterile: water only. Bacteriostatic: water plus 0.9% or 1.1% benzyl alcohol. |
|---|---|
| Multi-draw use | Sterile: single use. Bacteriostatic: multiple draws over a limited period per label. |
| Working life once entered | Sterile: use immediately. Bacteriostatic: days, per the product label. |
| Assay interference | Sterile: none. Bacteriostatic: benzyl alcohol can interfere with some analyses and cell-based work. |
| Storage | Both: controlled room temperature, 20-25 °C. Do not freeze. |
When each one is the right choice
Bacteriostatic water is the default for reconstituting a lyophilized research peptide that will be drawn on more than once. The preservative buys you a working window instead of forcing you to consume the vial in a single session.
Sterile water is correct where the preservative would be a problem: cell-based assays where benzyl alcohol is cytotoxic at working concentrations, analytical chemistry where it produces an interfering peak, or any single-use preparation where the preservative buys nothing.
Things worth knowing
- Benzyl alcohol is not inert. It is a real chemical entity with its own spectroscopic signature. If you are running mass spec or HPLC on a reconstituted sample, know that it is there.
- Bacteriostatic does not mean sterilising. It slows growth; it does not rescue poor aseptic technique.
- Neither is a substitute for the COA. Some compounds specify a particular vehicle. Where they do, that specification wins.
- Do not freeze either one. Freeze-thaw can compromise container integrity, and freezing the diluent achieves nothing useful.
Related
See our step-by-step guide to reconstituting lyophilized peptides, or browse bacteriostatic water in the catalog.
This article covers laboratory handling practice only. All materials supplied by Never Enough Labs are for in-vitro research use by qualified persons 21 years of age or older. Nothing here is medical, veterinary or diagnostic guidance, and no product described is intended for human or animal use.