Reconstitution
Bacteriostatic water
Bacteriostatic water is sterile water with 0.9% benzyl alcohol added as a preservative, allowing repeated withdrawals from the same vial.
Bacteriostatic water is Water for Injection, USP, with 0.9% benzyl alcohol added as a preservative. That single ingredient is the whole difference from plain sterile water: benzyl alcohol slows the growth of common bacteria inside the vial, which is why bacteriostatic water is labeled for multi-dose use — you can draw from the same vial more than once — rather than for a single withdrawal.
Why the preservative matters for reconstitution
When a lyophilized powder is dissolved for reconstitution, the resulting solution often needs to be drawn from repeatedly over days or weeks rather than used all at once. A preservative-free liquid opened and re-entered many times carries a higher contamination risk with each puncture. Bacteriostatic water is formulated for exactly that repeated-access pattern, which is why it’s the diluent named on most multi-dose peptide vial instructions.
Bacteriostatic water vs. sterile water vs. saline
- Bacteriostatic water — water plus 0.9% benzyl alcohol; multi-dose.
- Sterile water for injection — water with no preservative; labeled single-use.
- Saline (0.9% sodium chloride) — a salt solution, not a preserved water; a different diluent entirely, used where a product’s instructions specifically call for it.
Which diluent a given vial actually requires is set by that product’s own instructions, not by this definition. Once you know the diluent volume, the reconstitution chart helps you check the concentration produced by the vial amount and water volume; it does not tell you what to draw.
Use it
Common questions
What is bacteriostatic water?
Bacteriostatic water is Water for Injection, USP with 0.9% (9 mg/mL) benzyl alcohol added as a preservative. The benzyl alcohol inhibits the growth of common bacteria in the vial between uses, which is why it is labeled for multi-dose use rather than a single withdrawal.
What is bac water, and is it different from bacteriostatic water?
"Bac water" is just shorthand for bacteriostatic water — same product, same 0.9% benzyl alcohol preservative.
Is bacteriostatic water the same as sterile water?
No. Sterile water for injection contains no preservative and is labeled for single use only. Bacteriostatic water contains benzyl alcohol specifically so a vial can be entered more than once. Once a bacteriostatic water vial is opened, most labeling gives it a limited multi-dose window (commonly cited as up to 28 days) — check the specific product's label for its stated window.
Is bacteriostatic water the same as saline?
No. Saline (sodium chloride solution) contains dissolved salt at a specific concentration; bacteriostatic water is water with a preservative and no salt. They are different diluents used for different purposes and are not interchangeable without checking what a given product calls for.
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