Dr.PainCLE said:

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

I read this differently from Dr.PainCLE, 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.

I would rather be corrected than agreed with, if it comes to it.

KevinCompounds 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…

GMP compliance checklist relevant to compounded supply compounding quality:

• Clean room environment with HEPA filtration
• Environmental monitoring (viable and non-viable)
• Personnel training and competency assessment
• Validated analytical methods for testing
• Stability testing program
• Complete batch records and traceability
• Quality management system with deviation handling

A 503B pharmacy following cGMP should be able to answer questions about ALL of these if you ask. Don't be afraid to ask — it's your health.

Dr.SportsMedIN 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 mike_mealprep’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.