503A vs 503B

Two sections of the US Food, Drug and Cosmetic Act governing drug compounding — traditional patient-specific pharmacies under 503A, larger outsourcing facilities under 503B — that set which substances compounders may use.

Sections 503A and 503B of the US Federal Food, Drug and Cosmetic Act define two categories of drug compounding. A 503A pharmacy compounds medications for identified individual patients against prescriptions, under state pharmacy regulation. A 503B outsourcing facility registers with the FDA, may compound larger batches without patient-specific prescriptions, and is held to full pharmaceutical manufacturing standards.

The sections matter to the peptide field because they control inputs. A substance may generally be compounded only if it is a component of an approved drug, has an applicable pharmacopoeial monograph, or appears on the FDA's lists of bulk substances permitted for each section. Peptides that fit none of those routes cannot lawfully be compounded for human use, whatever their research literature says, and the FDA's category reviews of specific peptides — including the July 2026 advisory committee votes — are decisions about these lists.

None of this framework governs research-use-only supply, which sits outside compounding entirely; the connection is that 503A/503B decisions are frequently misread as verdicts on a compound's merits. The site's guide on the 2026 review sets out what such a decision does and does not mean.

SourceFDA — compounding laws and policies

For research purposes only. Not for human consumption, diagnosis, treatment, or prevention of any condition. Nothing on this page is medical advice.