SarahChen_PharmD said:

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

This is where I part company with the consensus forming above. 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.

alex_tucson 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…

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.

FDA_TrackerJim said:

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

Price comparison for compounded supply across sources I've used:

I use the 503B option — best balance of quality, cost, and legality. The brand price is insane but insurance can help if you qualify.

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

Why a shortage listing created a legal pathway at all, since a shortage is a supply fact rather than a permission?

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.