Collecting this in one place because it comes up every few weeks and the answer is always assembled from scratch. It is about compounded supply, and it is deliberately narrow — everything I am not confident about is marked as such.
What is actually established
Resolution therefore closed the doors unevenly, and the asymmetry follows from the bulks lists. For 503B the shortage clause was the only route to these molecules, so that route shut completely. A 503A pharmacy can still argue a doorway via "component of an approved drug" — but only for the substance in the form present in the approved product, which is exactly where the base-versus-salt argument lives, and it does nothing about the copy restriction, which came back into force on resolution.
The condition it depends on
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.
The practical version
Two things anyone can check: a state licence number for a 503A, and an FDA outsourcing-facility registration for a 503B. Both are publicly searchable, and a pharmacy unwilling to give you either has answered the question.
What I am not sure about
So the question, as narrowly as I can put it: why a shortage listing created a legal pathway at all, since a shortage is a supply fact rather than a permission. I have searched first, so if this is covered somewhere point me at it and I will read it.
— Dr.SurgeonPGH · corrections welcome and will be edited into this post with credit