A 503A compounding pharmacy is a state-licensed pharmacy that prepares custom medications for individual patients based on a prescription. They operate under strict regulations from state boards of pharmacy: sterile-compounding standards, facility controls, identity and potency verification, sterility and endotoxin testing by accredited third-party labs, and beyond-use dating on every vial.
503A pharmacies don't produce mass-market drug products. They prepare patient-specific medications, which is why their formulations are exempt from the standard FDA approval pathway under Section 503A of federal law. The safety comes from clinical oversight and pharmacy quality, not from a stock-keeping unit being FDA-listed.