Has anyone dealt with dumb question - what is bacteriostatic water exactly?

Dr.RenalNash said:

Dead space is the answer to the missing dose.

I am going to disagree with reconstituting low as a general rule. More diluent, more punctures, more in-use days at room temperature, and the stability trade-off is real. Precision is not the only variable being optimised.

NauseaFreeNow said:

Two of us with identical vials and identical diluent arrived at different unit counts, which means one of us has a conceptual error rather than a…

Compounding pharmacy dose accuracy check for dosing arithmetic: I always verify my first dose from a new vial by weighing it on a milligram scale. Draw up your dose, weigh the syringe, subtract empty syringe weight, compare to expected water weight at that volume.

For a 10-unit dose (0.1mL): expected weight ~100mg of water. If it's significantly off, your syringe may be inaccurate. Cheap insulin syringes can have ±5% volume accuracy. Spring for quality brands like BD or Terumo.

VendorMark said:

I am going to disagree with reconstituting low as a general rule.

Regarding dosing arithmetic multi-dose vial contamination prevention: after reconstituting, I wipe the vial stopper with a fresh alcohol swab before EVERY single draw. I also never touch the stopper with my fingers and use a new needle for each draw.

Even with bacteriostatic water preservative, contamination is cumulative. One sloppy draw can introduce bacteria that multiply over days. Aseptic technique isn't optional — it's essential for injectable medications.

A narrower follow-up, since the general answer is now clear:

How much material I am losing to dead space, and whether that explains why a 10mg vial gives me nine usable draws rather than ten?

Reporting back.

Follow-up: it was dead space. Switching to a low-dead-space fixed-needle syringe gave me the tenth dose back.