DataDave said:

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

I read this differently from DataDave, 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.

steve_okc 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…

For compounded supply users considering compounded for the first time: here's a step-by-step guide:

1. Get a prescription from your doctor (telehealth counts)
2. Research 503B compounding pharmacies with good community reviews
3. Verify their FDA registration and inspection history
4. Request a COA for your specific compound before ordering
5. Start with a 1-month supply to test
6. Consider sending a sample to Janoshik for independent verification
7. Track your response compared to brand (if you were on it previously)

PeptideChemSF 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 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.

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

What actually distinguishes 503A from 503B, in terms of what each may make and from what starting material?

Reporting back.

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.