This guidance is intended to help small businesses better understand and comply with the final rule establishing the list of bulk drug substances that can be used in compounding under section 503A of the Federal Food, Drug, and Cosmetic Act (FD&C Act) (21 U.S.C. 353a) (final rule or 503A Bulks List Final Rule). On February 19, 2019 (84 FR 4696), FDA published a final rule that establishes FDA’s criteria for evaluating bulk drug substances for inclusion on the list of bulk drug substances that may be used to compound drug products under section 503A of the FD&C Act (the 503A Bulks List or the list) and that places six substances on the list. The final rule, entitled “List of Bulk Drug Substances That Can Be Used To Compound Drug Products in Accordance With Section 503A of the Federal Food, Drug, and Cosmetic Act,” also identifies four substances that were considered for and not included on the list.