Dr.ReproEndo said:

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

Pushing back on Dr.ReproEndo here. 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.

One concrete data point for the thread. 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.

I would rather be corrected than agreed with, if it comes to it.

JessicaH_TX said:

A research-chemical supplier selling lyophilised powder labelled research use only is not compounding and is not claiming to.

Coming at JessicaH_TX’s question from a different direction. 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.

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?

OP back with an update, since a thread like this is useless without one.

Follow-up — I binned the frozen one. Aggregation does not undo itself and I was not going to inject a maybe.