Adding the clinical framing, because it changes how the question reads. The version of this that has an answer is narrower than the version being asked. Narrow it and it becomes tractable; leave it broad and the thread will produce nine confident and incompatible replies.

From the other side of the consultation, briefly. Order of operations matters more than any single choice here: establish a baseline, change one thing, wait long enough for it to express itself, then measure again under the same conditions.

LipidDoc_ATL said:

Order of operations matters more than any single choice here: establish a baseline, change one thing, wait long enough for it to express itself, then…

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.

LipidDoc_ATL said:

Order of operations matters more than any single choice here: establish a baseline, change one thing, wait long enough for it to express itself, then…

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

Moderator note: the sourcing question belongs in the vendor section and has been split out. Carry on.