DanielChem_CHI said:

Resolution therefore closed the doors unevenly, and the asymmetry follows from the bulks lists.

This is where I part company with the consensus forming above. A research-chemical supplier selling lyophilised powder labelled research use only is not compounding and is not claiming to. It is a different legal universe with no pharmacy oversight, no patient relationship and no content guarantee, and conflating the two in these threads helps nobody.

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

VanRx_Mike 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…

Price comparison for compounded supply across sources I've used:

I use the 503B option — best balance of quality, cost, and legality. The brand price is insane but insurance can help if you qualify.

amsterdam_pete said:

A research-chemical supplier selling lyophilised powder labelled research use only is not compounding and is not claiming to.

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.

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?

Reporting back.

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.