FDA_TrackerJim 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…

I have been on both brand and compounded, and in the context of compounded supply, my experience has been equivalent with both. The key is finding a reliable 503B pharmacy with independent testing.

One thing that is still open after james_edin’s answer:

What actually distinguishes 503A from 503B, in terms of what each may make and from what starting material?

Dr.RaviCardio said:

I have been on both brand and compounded, and in the context of compounded supply, my experience has been equivalent with both.

GMP compliance checklist relevant to compounded supply compounding quality:

• Clean room environment with HEPA filtration
• Environmental monitoring (viable and non-viable)
• Personnel training and competency assessment
• Validated analytical methods for testing
• Stability testing program
• Complete batch records and traceability
• Quality management system with deviation handling

A 503B pharmacy following cGMP should be able to answer questions about ALL of these if you ask. Don't be afraid to ask — it's your health.

Closing the loop on my own question.

The bulks-list asymmetry was the piece I had missed entirely. It explains why one of my two pharmacies is still arguing it can supply and the other simply stopped.

Dr.AddMedPHL said:

GMP compliance checklist relevant to compounded supply compounding quality: Clean room environment with HEPA filtration Environmental monitoring…

That is correct as far as it goes, and here is where it stops going. 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.