Dr.BariatricHTX said:

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

I read this differently from Dr.BariatricHTX, on substance rather than tone. 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.

LabKate 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…

Beyond-use dating (BUD) for compounded supply compounded products:

Lyophilized (unreconstituted): typically 6 months from compounding date
Reconstituted with BAC water: typically 28-30 days refrigerated
Reconstituted with sterile water: typically 24 hours (no preservative!)

Always use BAC water for multi-dose vials. Sterile water has no preservative and supports bacterial growth after first puncture. This is a common and potentially dangerous mistake.

COA_Karl said:

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

Adding the part of the answer the thread has not reached. The trade-off is reversibility against privacy and there is no option that gives you both. Card payments are reversible and disclose the most; crypto discloses least and is irreversible, which is precisely why pressure toward it is a warning sign when it comes from a seller rather than a buyer. Escrow only means anything where the escrow agent is independent of both parties, which is rarely the case in practice.

One thing that is still open after hank_denver’s answer:

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.