A question that comes up constantly: the dose has not changed, but the units drawn are different from last month. What happened?
Almost always, the vial happened.
Units are a volume measure. Milligrams are a mass measure. The bridge between them is concentration, and concentration is set the moment you decide how much bacteriostatic water goes into the powder. Change that decision and every unit count downstream changes with it, even though the dose in milligrams is identical.
Concretely. A 5 mg vial reconstituted with 1 mL gives 5 mg/mL. A 0.5 mg dose is 0.1 mL, which is 10 units on a U-100 syringe. Reconstitute the next 5 mg vial with 2 mL and you have 2.5 mg/mL. The same 0.5 mg dose is now 0.2 mL, which is 20 units. Nothing about the dose moved. The concentration halved, so the volume doubled.
This is why a log that records only units is a trap. Units are meaningful within one vial and meaningless across two. A log that records milligrams stays correct forever, and the unit count is derived from it whenever the vial changes. Keeping both, with the concentration noted alongside, is what lets you look back at an entry from three months ago and know what it meant.
The practical habit is to recompute at every new vial rather than assume. It takes under a minute in a reconstitution calculator, and it catches the case where you thought you added 2 mL and actually added 1.5.
Write the concentration on the vial in permanent marker. Not on a note, not in your head. On the vial, where it will be in front of your eyes at the moment you draw.