What water should be used to reconstitute a peptide?

Three sterile diluents are commonly appropriate for peptide reconstitution, each with a specific use case. Bacteriostatic water for injection (BAC water) contains 0.9% benzyl alcohol as a preservative that inhibits bacterial growth in the vial over several weeks of multi-dose access. This is the peptide-therapy default for lyophilized research-supply vials because most such vials are multi-dose.

Sterile saline (0.9% sodium chloride) contains no preservative, matches blood tonicity, and is preferred for single-use preparations, intranasal delivery where mucosa is sensitive to benzyl alcohol, and any context where preservative tolerability is a concern. Sterile water for injection (without preservative and without tonicity match) is a third option, but its lack of preservative combined with hypotonic osmolality make it inappropriate for most subcutaneous multi-dose use.

Certain compounds require compound-specific diluents. NAD+ is often supplied with pH-adjusted or buffered diluent because unbuffered NAD+ reconstitution produces injection-site pain from non-physiological pH. Glutathione may be supplied with a slightly acidic vehicle because it is more stable in that pH range. In all cases, use the diluent specified by the supplier or compounding pharmacy. Tap water is never appropriate — it is not sterile and contains minerals and dissolved organics that can react with peptide chemistry.