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Guide
Bacteriostatic water is sterile water with 0.9% benzyl alcohol added as a preservative. The benzyl alcohol stops bacteria from multiplying, which is why it is the standard diluent for reconstituting multi-dose peptide vials used over several weeks. This guide covers what it is, how it differs from sterile water, and how much to use.
Bacteriostatic vs sterile water
Sterile water for injection contains no preservative, so once a vial is entered it should be used quickly. Bacteriostatic water adds 0.9% benzyl alcohol, which suppresses bacterial growth and lets a reconstituted multi-dose vial be used over roughly 28 days when refrigerated. For a single-use preparation sterile water is common; for a peptide vial you will draw from repeatedly, bacteriostatic water is the usual choice.
How much bacteriostatic water to add
There is no single correct volume — the amount of water sets the concentration, not the dose. More water makes a more dilute solution that draws as more syringe units per milligram; less water makes it more concentrated. Pick a volume that puts your dose on an easy-to-read number of units. The bacteriostatic water calculator and the reconstitution solver work this out for you.
Storage and beyond-use date
Once mixed, keep the vial refrigerated at 36 to 46 °F (2 to 8 °C) and protect it from light. A commonly used beyond-use date for a reconstituted multi-dose vial is 28 days under USP Chapter 797, after which it should be discarded. Always follow the product’s Certificate of Analysis or label when it gives a specific figure.
Frequently asked questions
Is bacteriostatic water the same as sterile water?
No. Sterile water for injection has no preservative, while bacteriostatic water adds 0.9% benzyl alcohol so a multi-dose vial can be entered repeatedly over several weeks. For peptides you draw from more than once, bacteriostatic water is the usual choice.
How long does bacteriostatic water last once opened?
The bacteriostatic water vial is often labeled for up to 28 days after the first puncture — follow the manufacturer’s label. A peptide reconstituted with it also has its own beyond-use date, commonly 28 days refrigerated under USP Chapter 797.
Can I use tap water or regular distilled water?
No. Only sterile water or bacteriostatic water intended for injection should be used to reconstitute peptides. Tap, distilled, or spring water is not sterile and can introduce contamination into the vial.
Why does the benzyl alcohol matter?
Benzyl alcohol is the bacteriostatic agent: at 0.9% it suppresses bacterial growth inside the vial, which is what makes bacteriostatic water suitable for multi-dose use. It is generally avoided in newborns and in very large volumes.
Does the amount of bacteriostatic water change my dose?
No. The water sets the concentration, and therefore how many syringe units a dose measures, but the milligrams of peptide you draw for a given dose stay the same. Use the reconstitution tools to pick a convenient volume.