Bacteriostatic water
Also known as “bac water”
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Sterile water containing 0.9% benzyl alcohol, which inhibits bacterial growth so a reconstituted multi-dose vial stays usable for longer (commonly up to ~28 days refrigerated). The standard diluent for lyophilized peptides.
The distinction from plain sterile water is practical: the benzyl alcohol lets a single vial be entered repeatedly over roughly a month, which is why it is the default diluent for multi-dose peptide vials. The volume added is also the number that sets concentration, so choosing, for example, 2 mL versus 3 mL directly changes how many units each dose requires. It is not a drug in itself, only the carrier fluid.
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Educational reference only — not medical advice. Always follow the guidance of a qualified healthcare professional.