Bacteriostatic water ("BAC water") is the standard diluent for reconstituting freeze-dried peptides — sterile water with a benzyl-alcohol preservative.
The preservative lets one vial be drawn from repeatedly over days or weeks; plain sterile water has none and is single-use.
How much you add sets the concentration, not the amount of peptide — pick a volume that gives an easy-to-read draw.
Add the water down the vial wall, swirl gently (never shake), and label with concentration and date.
Store the reconstituted vial refrigerated and use it within its preservative window.
Bacteriostatic water — often shortened to "BAC water" — is the liquid used to reconstitute freeze-dried research peptides back into an injectable solution. It is sterile water with a small amount of benzyl alcohol (about 0.9%) added as a preservative, and that preservative is what makes it the standard choice for peptides1.
Why peptides use bacteriostatic water
Research peptides arrive as a dry, freeze-dried powder that has to be dissolved before it can be drawn into a syringe. Bacteriostatic water is preferred over plain sterile water for one reason: its benzyl-alcohol preservative suppresses microbial growth, so a single vial can be entered and drawn from repeatedly over the days or weeks it takes to use it1. Plain sterile water has no preservative and is intended for single use. For the full chemistry, see what bacteriostatic water is; for how the term compares to "reconstitution solution", see reconstitution solution vs bacteriostatic water.
How much bac water to use
There is no single "correct" amount — the volume you add sets the concentration, not the amount of peptide. Adding more water makes a weaker solution (more units per dose); adding less makes a stronger one. A common approach is to pick a round concentration that makes the syringe easy to read.
There is no fixed amount — the water volume sets the concentration, not the amount of peptide. Adding 2 mL to a 10 mg vial gives 5 mg/mL; 1 mL gives 10 mg/mL. Choose a volume that makes your draw easy to read and confirm it in the calculator.
Can you reuse a bacteriostatic water vial?
Yes — that is the point of the benzyl-alcohol preservative. A multiple-dose vial can be entered repeatedly over its labelled window, as long as the stopper is swabbed each time and it is stored as directed.
What can I use instead of bacteriostatic water?
Plain sterile water will dissolve a peptide, but it has no preservative, so the vial becomes single-use once entered. Bacteriostatic water is preferred precisely because it allows repeated draws over days.
References
Pfizer Injectables / U.S. Pharmacopeia Bacteriostatic Water for Injection, USP — prescribing information (0.9% benzyl alcohol added as a bacteriostatic preservative; multiple-dose container).DailyMed (U.S. National Library of Medicine). 2023. Bacteriostatic Water for Injection USP
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