Dr.SurgeonPGH said:

The shortage clause is the answer to the second question and it is a subtraction rather than an addition.

Pushing back on Dr.SurgeonPGH here. 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.

mona_PHX said:

My pharmacy stopped supplying with three weeks notice and a letter that explained nothing, so I went and read the statute.

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.

mike.trainer_LA said:

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

Compounded has been just as effective for me. If compounded supply is the concern, the COA from my 503B pharmacy shows 99.1% purity. Verified by Janoshik.

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.