Short answer: The difference is the preservative. Bacteriostatic water contains a small amount of benzyl alcohol that inhibits bacterial growth (as stated on USP Bacteriostatic Water for Injection labeling), so a vial can be entered multiple times over several weeks. Sterile water has no preservative — it's sterile when sealed but has no ongoing protection once opened, making it single-use.
TL;DR — Bacteriostatic water = has a preservative → multi-use, lasts ~28 days once opened. Sterile water = no preservative → single-use. For reconstituting research peptides that you'll draw from more than once, bacteriostatic water is the standard choice.
Bacteriostatic water
- Contains ~0.9% benzyl alcohol, a bacteriostatic (growth-inhibiting) preservative.
- Because the benzyl alcohol inhibits the growth of most bacteria, a single vial can be accessed multiple times — the labelled in-use window is up to 28 days after first entry. Note that the 28 days is not conditional on refrigeration: USP Bacteriostatic Water for Injection labeling specifies controlled room temperature storage (20–25 °C). Benzyl alcohol toxicity in neonates is a separate, well-documented concern in the pharmaceutical literature (e.g. animal and clinical reports), which is one reason preservative-free sterile water is preferred when a protocol prohibits preservatives. Bacteriostatic means bacterial growth is inhibited, not that the vial resists contamination.
- This makes it the preferred solvent for reconstituting lyophilized research peptides, which are usually used across several sessions.
Sterile water
- Simply purified water that has been sterilized, with no preservative added.
- Sterile when the vial is sealed, but once opened there is nothing to inhibit microbial growth, so it is intended for single use.
- Better suited to one-time preparations rather than a vial you'll return to repeatedly.
Which to use for reconstitution
For most research peptide work — where a reconstituted vial is drawn from more than once — bacteriostatic water is the standard because its preservative keeps the solution usable over the working period. If a protocol specifically requires no preservative, sterile water may be used, understanding it should be treated as single-use.Quick comparison
- Preservative: Bacteriostatic = yes (benzyl alcohol) · Sterile = no
- Reuse: Bacteriostatic = multi-use (~28 days) · Sterile = single-use
- Best for: Bacteriostatic = reconstituting research peptides · Sterile = one-time preparations