Dr.GastroMayo said:

They are two different exemptions from the same federal requirements and they buy different things.

Filing a mild objection. Mild because I might be wrong; an objection because nobody has addressed the case that does not fit. 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.

Correct me if the detail matters more than I have assumed.

WendyG_ATL said:

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

Compounding pharmacy customer here with experience relevant to compounded supply.

I've ordered from 5 different compounding pharmacies over the past 18 months. The quality variation is real — purity ranged from 94% to 101% of label claim based on independent Janoshik testing.

My current compounder (a 503B facility) has been consistently 98-101% purity across 5 orders. I pay $136/month vs $1,346 for brand. The savings are substantial and the product is equivalent in my experience.

Re: compounded supply — this applies whether you're using brand or compounded. The clinical principles are the same.

DebRD_ATL 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?

OP back with an update, since a thread like this is useless without one.

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.