One concrete data point for the thread. The three numbers to write on the vial: total mg, total ml, and mg per ml. Everything else is division. And the sanity check is that dose volume times number of doses should be less than the volume you put in, because dead space takes the difference.
Worth separating that from the dosing arithmetic, which this thread keeps folding into the same question. They behave differently and the advice does not transfer.
That is the short version; the long version is somebody else's post.
A narrower follow-up, since the general answer is now clear:
Why a shortage listing created a legal pathway at all, since a shortage is a supply fact rather than a permission?
BethLabQueen said:The three numbers to write on the vial: total mg, total ml, and mg per ml.
Adding the part of the answer the thread has not reached. Do it in two steps and it stops being confusing. First concentration: 10mg into 2ml is 5mg/ml. Then volume: a 0.5mg dose is 0.5 ÷ 5 = 0.1ml. Then units, and this is where people go wrong — a U-100 syringe is graduated in hundredths of a millilitre, so 0.1ml is 10 units. The word "units" has nothing to do with milligrams; it is a volume marking that exists because insulin happens to come at 100 units per ml.
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The bulks-list asymmetry was the piece I had missed entirely. It explains why one of my two pharmacies is still arguing it can supply and the other simply stopped.
Dr.PeteFamMed said:Do it in two steps and it stops being confusing.
Agreeing with Dr.PeteFamMed, and the qualification matters more than the agreement. 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.
Worth separating that from the dosing arithmetic, which this thread keeps folding into the same question. They behave differently and the advice does not transfer.