Bacteriostatic water is the liquid most people use to reconstitute peptides. It is water with about 0.9% benzyl alcohol added, and that preservative is the main reason a mixed vial can be kept and used over several weeks rather than a single sitting.
Reviewed for accuracy · Last reviewed July 7, 2026Sterile water and bacteriostatic water are both sterile at the point of manufacture. The difference is that bacteriostatic water contains benzyl alcohol, which suppresses the growth of many bacteria after the vial has been entered multiple times. Sterile water has no preservative, so it is intended for a single use.
For a peptide vial that you will draw from repeatedly over weeks, bacteriostatic water is the common choice for that reason. Some peptides and some people react to benzyl alcohol, which is one reason sterile water is occasionally used instead.
There is no single right amount of water. Adding more makes a weaker, larger-volume solution that is easier to measure for small microgram doses; adding less makes a stronger solution so a higher dose fits in one syringe. What matters is calculating the resulting concentration so your dose is accurate.
Work the concentration out with the calculator, or by hand: peptide amount divided by water volume gives mg/mL.
An unopened vial of bacteriostatic water and a reconstituted peptide are both kept refrigerated and out of direct light. The benzyl alcohol is what lets a mixed vial last, but it does not make it last forever, and different peptides degrade at different rates. Treat the compound-specific storage window as the limiting factor.