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.

BethLabQueen said:

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

Agreed, and the enforcement dates were staggered by category — 503A first, 503B a few weeks later — because outsourcing facilities have manufactured inventory and clinic contracts to unwind while a 503A makes to order.

PharmD_Rodriguez said:

Two pharmacies quoted me last year, one describing itself as 503A and one as 503B, and I assumed 503B just meant bigger until both stopped within…

Can confirm the pattern PharmD_Rodriguez describes. 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.

Clinical perspective, offered as context rather than as advice.

Compounding pharmacy response time test for compounded supply: I email customer service at odd hours to test responsiveness. A pharmacy that can't answer questions promptly is a red flag.

My current pharmacy: average response time 1 hour. They answered my questions about batch-specific COAs thoroughly and professionally.

Communication quality is a proxy for operational quality. A pharmacy that communicates well is likely manufacturing well too.