Answering the narrow version, because the broad one does not have a single answer. 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.

Dr.CardioMD said:

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

Dr.CardioMD has the substance of this right. The condition it depends on is worth stating. Worth adding the genuine exception, because it is real and narrow: a change made for an identified patient where the prescriber determines it produces a significant clinical difference for that patient. A grid of fixed doses offered to everybody is not that, whatever the intake form says.

Admin said:

One order arrived in nine days, one was held at the border for a fortnight, and one came back with a seizure notice.

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.

Ask again with the specifics and you will get a better answer than this one.

Clinical perspective, offered as context rather than as advice.

International shipping tracking for cross-border ordering orders: my experience with customs in the US when ordering from overseas:

Package cleared customs in 2 days, no issues.

Tips: always include a copy of your prescription, declare medication properly on customs forms, keep quantities reasonable (personal use = 3 month supply max), and use a vendor who has experience shipping to your country.