What it is
Bacteriostatic water is plain sterile water with one addition: about 0.9% benzyl alcohol, a mild preservative. “Bacteriostatic” means it holds bacteria static — it doesn’t kill microbes, but it stops them from multiplying, which is what makes a single vial safe to use across multiple doses for roughly 28 days after it’s first punctured.
Where it comes up with GLP-1s
You’ll only encounter bacteriostatic water in the compounded corner of the GLP-1 world. Compounded semaglutide or tirzepatide is often supplied either as a powder that must be reconstituted, or as a concentrate that’s diluted before injecting — and bacteriostatic water is the standard fluid for both. Brand-name products don’t need it, because their pens arrive pre-filled and ready.
Getting the mixing right is where errors happen, and it’s why the arithmetic matters: the volume of water you add determines the concentration, which determines how many units equal your intended dose. The reconstitution calculator does that math. Handling and sourcing sit inside the broader 503B pharmacy and compounding rules, which are worth understanding before going this route.