labquiet_amy said:

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

I read this differently from labquiet_amy, 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.

jason_sac26 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…

FDA inspection results for compounding pharmacies selling compounded supply: you can look up any 503B facility's FDA inspection history on the FDA website.

Check for: Form 483 observations (violations), warning letters, and recall history. A clean inspection record is a strong indicator of quality. My pharmacy has had 0 Form 483 observations in their last 4 inspections.

This is public information. Use it. Link: fda.gov/inspections-compliance-enforcement-and-criminal-investigations

Dr.LipidDallas 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.

Following on from tyler_CSCS — 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?

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.