The short answer
A peptide is a short chain of amino acids — the same building blocks that make up proteins, just fewer of them strung together. There's no hard cutoff, but generally a chain of roughly 2 to 50 amino acids is called a peptide; longer chains are called proteins.
Your body makes thousands of them naturally. Insulin, the hormone that regulates blood sugar, is a peptide. So is oxytocin. So are the growth-hormone-releasing signals your pituitary gland responds to every night while you sleep.
Why peptides are having a moment
Two things changed recently. First, semaglutide and tirzepatide — both peptides — became blockbuster weight-loss drugs, putting the word "peptide" into everyday conversation. Second, synthetic peptide manufacturing got dramatically cheaper, making it possible for smaller labs (and unregulated vendors) to produce experimental peptides that would once have been confined to university research.
That second part is why this site exists: there's now a wide gap between what's rigorously studied and what's easy to buy.
Peptide, hormone, protein — what's the difference?
These categories overlap more than they're distinct. A hormone is defined by what it does (signals between cells or organs), not what it's made of — some hormones are peptides (insulin, GLP-1), others are steroids (testosterone, cortisol). A protein is really just a longer peptide, folded into a specific 3D shape that gives it a specific job (an enzyme, a structural component, an antibody).
So when you see "peptide" on this site, think: a small, engineered or naturally-occurring signaling molecule — not automatically a drug, not automatically safe, and not automatically well-studied just because it's "natural."
Why route of administration matters
Most peptides can't be taken as a pill, because stomach enzymes break them down before they can act — which is why almost everything on this site is studied as an injection, a nasal spray, or (for a few, like GHK-Cu) a topical application. This is a real pharmaceutical engineering problem, and it's part of why oral GLP-1 drugs took years longer to develop than the injectable versions.