Key takeaway
Peptides are short chains of amino acids that act as signalling molecules in the body. This primer explains their chemistry, how they differ from proteins, and why regulatory status varies so widely.
What exactly is a peptide?
A peptide is a short chain of amino acids linked by peptide bonds — conventionally fewer than about 50 residues, above which a chain is usually called a protein. Insulin (51 residues) sits near that boundary, which is why definitions are approximate rather than absolute.
In the body, endogenous peptides act as hormones and signalling molecules: they bind receptors and trigger downstream effects. Synthetic peptides aim to mimic or modify those signals.
Why does regulatory status differ so much between peptides?
Approval depends on whether a peptide has completed the clinical-trial pathway for a specific indication. Semaglutide and tirzepatide cleared large phase-3 programs and are approved medicines; BPC-157 and TB-500 have only preclinical evidence and are sold as research chemicals.
This is the single most important distinction for readers: "a peptide exists in the literature" is not the same as "a peptide is approved or proven in humans".
How should research evidence be weighed?
A useful hierarchy runs from in-vitro work, to animal models, to small human studies, to large randomized controlled trials. Most research peptides sit at the lower rungs. Throughout this encyclopedia we flag where evidence sits on that ladder and link the primary source.
Frequently asked questions
They are chemically similar but distinguished by length — peptides are short chains (conventionally under ~50 amino acids), proteins are longer and fold into complex structures.