mike.trainer_LA said:

The shortage clause is the answer to the second question and it is a subtraction rather than an addition.

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.

PeptideChemSF 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…

I have been on both brand and compounded, and in the context of compounded supply, my experience has been equivalent with both. The key is finding a reliable 503B pharmacy with independent testing.

TrialTracker_MD said:

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

Compounded semaglutide formulations for compounded supply: some pharmacies add ingredients like B12, L-carnitine, or BPC-157 to their semaglutide preparations. Are these beneficial or marketing gimmicks?

My take: B12 addition has some logic (GLP-1s can deplete B12). L-carnitine evidence is weak. BPC-157 for GI protection is theoretically interesting but unproven. I prefer straight semaglutide with no additives — fewer variables, cleaner data on what's working.

Following on from mike_mod — 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.