What are peptides?
Short answer · as of 2026-10-05
A peptide is a short chain of amino acids. FDA treats a chain of 40 or fewer amino acids as a peptide and a longer one as a protein; NIH's genetics glossary describes a peptide as typically 2 to 50, so thresholds differ by source. Some peptides are FDA-approved drugs. The word “peptide” says nothing about whether a product is approved, tested, safe or legal to sell.
By Peptide Industry Gossip · Published · General explanation, not legal or medical advice. Disclaimer.
The definition
Peptides and proteins are both chains of amino acids. FDA's 2020 final rule defines a protein, for the purposes of its biological-product regulation, as an alpha amino acid polymer with a specific, defined sequence that is greater than 40 amino acids in size. FDA's own policy document says it considers a polymer of 40 or fewer amino acids a peptide and not a protein.
NIH's National Human Genome Research Institute glossary describes a peptide as a short chain of amino acids, typically 2 to 50. The 40-amino-acid line is FDA's regulatory threshold, and other sources draw it differently.
Some peptides are approved drugs
FDA's 2021 guidance on synthetic peptide generics names five approved peptide drugs: glucagon, liraglutide, nesiritide, teriparatide and teduglutide. Being a peptide does not stop a product from being an approved drug, and it does not make it one.
What the word does not tell you
- Whether a product is FDA-approved. Approval is specific to a product and its use.
- Whether a seller's product contains what the label says. See our COA explainers.
- Whether it is legal to sell for human use. FDA's warning letters to research-use-only sellers address that question.
Sources
- Federal Register, Definition of the Term “Biological Product” (final rule, Feb. 21, 2020)
- FDA, MAPP 5016.3 Rev. 1: Classification of products as peptides or proteins
- NHGRI, Talking Glossary of Genomic and Genetic Terms: Peptide
- FDA, ANDAs for Certain Highly Purified Synthetic Peptide Drug Products That Refer to Listed Drugs of rDNA Origin (May 2021)