NurseKim_ATL said:

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

Pushing back on NurseKim_ATL 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.

PharmHunterJen 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…

503A vs 503B compounding pharmacies for compounded supply — this distinction matters enormously:

I strongly recommend 503B facilities. The FDA oversight and cGMP requirements mean more consistent product quality.

Dr.NutriCornell 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?

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.