Taking the question as asked, rather than the general version of it. 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.

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

Dr.ObesityMed said:

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

No disagreement with Dr.ObesityMed. One condition attached. 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.

raj_cambridge 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.

Clinical perspective, offered as context rather than as advice.

My experience ordering cross-border ordering internationally: I've ordered from an Australian compounder. Customs can be hit or miss.

Shipping time: 5-10 days
Customs clearance: was seized once, reshipped free
Quality: Janoshik tested at 97.3% purity

International ordering adds complexity but can save 60-80% compared to US prices. Research your country's import laws first.