503A Category 2

Section 503A of the US Federal Food, Drug, and Cosmetic Act creates the regulatory pathway under which a licensed pharmacist or licensed physician may compound a drug product for an identified individual, based on a valid prescription, using bulk drug substances that meet specific statutory conditions. One of those conditions is that the bulk drug substance either have a USP or NF monograph, be the component of an FDA-approved drug, or appear on a list - the so-called 503A Bulks List - developed by FDA after nomination and scientific review. The statute is codified at 21 U.S.C. 353a.

What "Category 2" means on the nominations list

FDA sorts nominated bulk drug substances into categories while review is ongoing. Category 1 substances are eligible for use in compounding while FDA evaluates the nomination. Category 2 substances are those that FDA has identified as raising significant safety risks and that it has therefore indicated should not be used in compounding pending further review; Category 3 substances have been nominated without sufficient supporting information for FDA to evaluate them. The working categories and the rationale for each are described in FDA's Interim Policy on Compounding Using Bulk Drug Substances Under Section 503A, 2018 (and updates). FDA's specific placement decisions for individual substances are published in the Federal Register and on FDA's Bulks List web page (FDA 503A Bulks List pages).

Why the distinction matters for the peptide landscape

Several peptides that circulate widely as research reference materials have been nominated for the 503A Bulks List and placed by FDA into Category 2, meaning the agency has taken the position that those substances should not be used in compounding under 503A pending further action. The published Federal Register notices and FDA review documents for those specific substances set out the agency's rationale in each case. For the purposes of a research-materials index, the Category 2 status is factual regulatory information about a peptide's status in the US compounding framework; it is not a statement about the peptide's utility as a biochemical reagent, which is a separate question governed by the RUO framework.

How Open Assay surfaces 503A status

Open Assay includes the current FDA 503A Bulks List status for each peptide in its database when that peptide appears on the list or in a nomination decision, cites the specific Federal Register or FDA document of record, and notes the date of the decision. Changes to the list are tracked over time. This is presented as reference information about the US regulatory landscape surrounding the peptide, not as guidance about compounding or any other downstream use.