LabKate said:

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

Agreeing with LabKate, and the qualification matters more than the agreement. They are two different exemptions from the same federal requirements and they buy different things. A 503A pharmacy is regulated primarily by the state board, needs a patient-specific prescription, is exempt from CGMP, and may use a bulk substance that has a USP monograph, is a component of an approved drug, or appears on the 503A bulks list — three independent doorways. A 503B outsourcing facility registers with the FDA, is inspected on a risk basis, must comply with CGMP, may compound for office stock without a patient-specific prescription, and has one doorway to a permitted bulk substance: the 503B bulks list, or the drug shortage list.

LabKate said:

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

I read this differently from LabKate, on substance rather than tone. 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.

Short answer first, then the reasoning. 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.

FDA_TrackerJim said:

They are two different exemptions from the same federal requirements and they buy different things.

Agreed, though "tolerable" needs defining. A dose you tolerate by eating almost nothing is not tolerated, it is being paid for somewhere else — usually in lean mass, sometimes in adherence three months later.

That is the short version; the long version is somebody else's post.