sophie_paris said:

The shortage clause is the answer to the second question and it is a subtraction rather than an addition.

That is correct as far as it goes, and here is where it stops going. Worth adding the genuine exception, because it is real and narrow: a change made for an identified patient where the prescriber determines it produces a significant clinical difference for that patient. A grid of fixed doses offered to everybody is not that, whatever the intake form says.

MeganSA_TX said:

The arithmetic is right and one caveat matters: a U-40 syringe changes the markings, not the volumes.

Genuinely useful, thank you. I had the facts and not the framework. I will report back once I have actually tried it.

sophie_paris said:

The shortage clause is the answer to the second question and it is a subtraction rather than an addition.

Filing a mild objection. Mild because I might be wrong; an objection because nobody has addressed the case that does not fit. A research-chemical supplier selling lyophilised powder labelled research use only is not compounding and is not claiming to. It is a different legal universe with no pharmacy oversight, no patient relationship and no content guarantee, and conflating the two in these threads helps nobody.

paul_denver 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.

Moderator note: the sourcing question belongs in the vendor section and has been split out.