This one has a reasonably settled answer, so here it is. 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.

RetaRick_CA said:

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

Agreeing with RetaRick_CA, and the qualification matters more than the agreement. 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.

hannah_MT 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…

This matches mine closely enough to be worth saying so. 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.

From the other side of the consultation, briefly.

For compounded supply users considering compounded for the first time: here's a step-by-step guide:

1. Get a prescription from your doctor (telehealth counts)
2. Research 503B compounding pharmacies with good community reviews
3. Verify their FDA registration and inspection history
4. Request a COA for your specific compound before ordering
5. Start with a 1-month supply to test
6. Consider sending a sample to Janoshik for independent verification
7. Track your response compared to brand (if you were on it previously)