Bacteriostatic water: what it is, and how much to use
4 min read
Bacteriostatic water is the liquid most freeze-dried peptides are mixed with. It is the one input that decides how concentrated the mixed vial ends up, and it cannot be changed once the powder is wet — so it is worth understanding before the needle goes near the vial.
What it is
Bacteriostatic water is sterile water with about 0.9% benzyl alcohol added. The benzyl alcohol is a preservative: it stops microbial growth, which is what lets a mixed vial be entered many times and kept for weeks rather than used once and discarded.
How it differs from sterile water and saline
Sterile water for injection is preservative-free, so a vial mixed with it is a single-use vial. Normal saline is sterile water with 0.9% sodium chloride and, again, usually no preservative; the salt also changes the solution. Neither is interchangeable with bacteriostatic water when the plan is to draw from the same vial over several days.
How much to use
The volume is a free choice, and it sets the concentration: concentration is the amount in the vial divided by the water added. More water gives a lower concentration and a larger, easier-to-measure draw; less water gives a higher concentration and a smaller draw. The amount delivered per dose does not change with the water — only how many units that dose lands on.
Storing a mixed vial
A mixed vial keeps in the fridge, roughly 2–8°C, out of the door and out of light. Wipe the stopper with a fresh alcohol pad before every draw. The preservative is what makes that multi-day use reasonable rather than the single use plain sterile water implies.