Bacteriostatic water — what it actually is, and why it isn't the same as sterile water
Comes up constantly and the answers are usually half right, so here's the full version.
**Bacteriostatic water** is sterile water containing about 0.9% benzyl alcohol as a preservative. Benzyl alcohol is *bacteriostatic* — it inhibits bacterial growth rather than killing what's already there. That distinction is in the name and it's the whole point.
**Sterile water for injection** has no preservative. It's sterile at the moment it's opened and has no ongoing protection after that.
Why it matters: the preservative is what makes repeated entry into the same vial reasonable. With no preservative, every entry is a fresh contamination opportunity and nothing suppresses growth afterwards. That's why single-use presentations exist.
**Reconstitution solution** is a separate commercial category, not a synonym. Formulations vary — some are effectively bacteriostatic water under another name, some contain different preservatives or buffers. If it doesn't state its composition, you don't know which.
One genuine caveat worth knowing: benzyl alcohol is not universally compatible. It's contraindicated in neonates, and some peptides are less stable in its presence. "Bacteriostatic water is always the right choice" is too strong a claim.
Evidence tier: this is established pharmaceutical chemistry, not peptide-specific research. It's the most solid ground on this forum.