COA_Karl said:

My pharmacy stopped supplying with three weeks notice and a letter that explained nothing, so I went and read the statute.

COA_Karl has the substance of this right. The condition it depends on is worth stating. 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.

COA_Karl said:

My pharmacy stopped supplying with three weeks notice and a letter that explained nothing, so I went and read the statute.

This is where I part company with the consensus forming above. 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.

Correct me if the detail matters more than I have assumed.

Answering the narrow version, because the broad one does not have a single answer. The shortage clause is the answer to the second question and it is a subtraction rather than an addition. Both exemptions forbid compounding something that is essentially a copy of a commercially available approved product. A product FDA has listed as in shortage is not treated as commercially available, so listing removed the objection that otherwise blocked compounding. It never created a permission; it withdrew a prohibition, which is why it evaporated the moment the supply fact changed.

LipidDoc_ATL said:

Resolution therefore closed the doors unevenly, and the asymmetry follows from the bulks lists.

Agreed, with a caveat about community reputation: it is a lagging indicator. Reports arrive weeks after orders, so a supplier can look excellent for a month after quality has already changed.

Ask again with the specifics and you will get a better answer than this one.