The two regulatory categories for compounding pharmacies in the United States, established by the Drug Quality and Security Act (DQSA) of 2013.

A 503A pharmacy operates under state board of pharmacy oversight, compounds medications based on individual patient prescriptions, and generally prepares small quantities. A 503B outsourcing facility registers with the FDA, may compound without patient-specific prescriptions, can produce larger batches, and must follow current good manufacturing practice (CGMP) standards including regular FDA inspections.

This distinction is directly relevant to peptide access. When a peptide drug appears on the FDA drug shortage list, both 503A and 503B facilities may compound copies. When the shortage ends, only 503A pharmacies with valid individual prescriptions may continue. The 503A versus 503B question has been central to the semaglutide and tirzepatide compounding access debate.