SarahChen_PharmD 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…

Compounding pharmacy response time test for compounded supply: I email customer service at odd hours to test responsiveness. A pharmacy that can't answer questions promptly is a red flag.

My current pharmacy: average response time 1 hour. They answered my questions about batch-specific COAs thoroughly and professionally.

Communication quality is a proxy for operational quality. A pharmacy that communicates well is likely manufacturing well too.

A narrower follow-up, since the general answer is now clear:

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

PharmacoVig_BOS said:

Compounding pharmacy response time test for compounded supply: I email customer service at odd hours to test responsiveness.

Compounding pharmacy licensing verification for compounded supply: before ordering, verify your pharmacy's credentials:

1. State Board of Pharmacy license — check on your state BOP website
2. FDA registration (for 503B) — check on fda.gov
3. DEA registration if handling controlled substances
4. PCAB accreditation — voluntary but demonstrates commitment to quality
5. USP <797>/<800> compliance for sterile compounding

This takes 10 minutes of research and could save you from receiving substandard or unsafe medication.

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.

hans_munich said:

Compounding pharmacy licensing verification for compounded supply: before ordering, verify your pharmacy's credentials: State Board of Pharmacy…

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.