labquiet_amy 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.

Ask again with the specifics and you will get a better answer than this one.

mel_PDX said:

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

Compounding pharmacy red flag checklist for compounded supply — if ANY of these apply, find a different source:

1. No verifiable physical address
2. No pharmacist available for consultation
3. COA not available or clearly template/fake
4. Prices dramatically below market ($30-40/month = suspicious)
5. No prescription required
6. Ships without cold pack or temperature control
7. No batch/lot numbers on product labeling
8. Pushes "research use only" products for human injection

Your health depends on product quality. Don't cut corners on source verification.

Dr.KarenChen said:

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

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

Lyophilized (unreconstituted): typically 12 months from compounding date
Reconstituted with BAC water: typically 28-30 days refrigerated
Reconstituted with sterile water: typically 48 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.

Following on from lori_vegas — and this may be the naive question:

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.