This one has a reasonably settled answer, so here it is. They are two different exemptions from the same federal requirements and they buy different things. A 503A pharmacy is regulated primarily by the state board, needs a patient-specific prescription, is exempt from CGMP, and may use a bulk substance that has a USP monograph, is a component of an approved drug, or appears on the 503A bulks list — three independent doorways. A 503B outsourcing facility registers with the FDA, is inspected on a risk basis, must comply with CGMP, may compound for office stock without a patient-specific prescription, and has one doorway to a permitted bulk substance: the 503B bulks list, or the drug shortage list.

That is the short version; the long version is somebody else's post.

Dr.SurgeonPGH said:

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

Agreeing with Dr.SurgeonPGH, and the qualification matters more than the agreement. The distinction that matters is unopened versus in use. Unopened vials are stable refrigerated for their labelled shelf life and tolerate a limited excursion to room temperature. Once reconstituted, the clock is much shorter and is set by the preservative rather than by the peptide — bacteriostatic water's benzyl alcohol is what buys you multiple draws over weeks. Sterile water has no preservative and turns a multi-dose vial into a single-use one.

LeilaHI said:

I froze a spare vial thinking I was being careful and later found out that is the one thing you are not supposed to do.

Agreed, and the enforcement dates were staggered by category — 503A first, 503B a few weeks later — because outsourcing facilities have manufactured inventory and clinic contracts to unwind while a 503A makes to order.

If somebody has the primary source to hand I would rather cite it than paraphrase it.

Clinical perspective, offered as context rather than as advice.

Storage best practices for compounded storage and cold:

• Unreconstituted lyophilized powder: refrigerate (36-46°F), good for 12+ months
• Reconstituted vials: refrigerate, use within 28 days
• Pre-filled pens: refrigerate until first use, then room temp OK for 28 days
• Never freeze reconstituted peptides — ice crystals damage the protein
• Protect from light — store in original box or wrap in foil

When in doubt, refrigerate. Temperature abuse is the #1 cause of reduced potency in compounded peptides.