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
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.
The condition it depends on
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.
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
The question I want answered is what actually distinguishes 503A from 503B, in terms of what each may make and from what starting material. Tell me what I have not thought of.
PurityPaulOR said:They are two different exemptions from the same federal requirements and they buy different things.
PurityPaulOR 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.
PurityPaulOR said:They are two different exemptions from the same federal requirements and they buy different things.
Pushing back on PurityPaulOR 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.
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Shop Reference StandardsShort answer first, then the reasoning. 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.
anders_CPH said:Resolution therefore closed the doors unevenly, and the asymmetry follows from the bulks lists.
Second this.