503A Pharmacy: These are traditional compounding pharmacies that prepare medications based on specific prescriptions for individual patients. They are regulated by state boards of pharmacy and must comply with USP <795>, <797>, and <800> standards. 503A pharmacies typically compound medications in smaller quantities, catering to unique patient needs.

503B Pharmacy: These are outsourcing facilities registered with the FDA under section 503B of the Federal Food, Drug, and Cosmetic Act. 503B pharmacies must follow Current Good Manufacturing Practices (CGMP), which are the same rigorous guidelines used by large pharmaceutical manufacturers. They produce medications in bulk and distribute them to healthcare facilities without requiring individual patient prescriptions. 503B facilities are also subject to regular FDA inspections, ensuring higher standards for sterility, quality, and consistency.