This guidance sets forth the Food and Drug Administration’s (FDA or the Agency) interim regulatory policy concerning compounding by outsourcing facilities registered under section 503B of the Federal Food, Drug, and Cosmetic Act (FD&C Act or Act) 2 using bulk drug substances. Section 503B of the FD&C Act includes certain restrictions on the bulk drug substances that outsourcing facilities can use in compounding and directs FDA to develop a list of bulk drug substances that can be used in compounding under that section. FDA is developing that list of bulk drug substances (the 503B bulks list), and this guidance describes FDA’s interim regulatory policy regarding outsourcing facilities that compound human drug products using bulk drug substances while the list is being developed.