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A peptide is a short chain of amino acids — the same building blocks proteins are made of, just fewer of them linked together. Most of the body’s signaling molecules (insulin, oxytocin, growth-hormone releasing hormone, and many others) are peptides; they tell cells what to do by binding to specific receptors on the cell surface. The compounds catalogued on this site are either copies of natural human peptides or engineered variants designed to last longer, bind more selectively, or resist being digested.
The same answer at three reading levels. Open whichever one matches what you actually want to know.
Your body is built out of tiny lego pieces called amino acids. There are about 20 different shapes. When the body snaps a bunch of them together in the right order, it builds proteins — the stuff your hair, muscles, and skin are made from. A long lego chain is a protein.
A peptide is what we call a really shortlego chain. Maybe ten pieces. Maybe thirty. Short enough that it’s not really a building yet — it’s a message.
Your body uses these little messages all day long to tell cells what to do. A peptide might say:
Every message has a specific receiver. Cells have little locks on their surface, and each peptide is a key shaped to fit just one kind of lock. Wrong key, nothing happens. Right key, the cell does whatever the message says.
The compounds on this site are either copies of messages your body already makes, or new messages that scientists designed to do something useful — like “repair this tendon faster” or “eat less without feeling sad about it.”
One catch: if you swallow a peptide, your stomach treats it like food and chops it up before it can deliver the message. That’s why most of them have to be injected — to skip the stomach entirely and get into the bloodstream intact.
The body uses 20 amino acids as its primary building blocks. Each one has the same backbone (a central carbon attached to an amino group —NH₂, a carboxyl group —COOH, and a hydrogen) plus a unique side chain that gives it different properties: some are positively charged, some negatively charged, some hydrophobic, some can form hydrogen bonds.
When two amino acids join, the amino group of one reacts with the carboxyl group of the next, releasing a water molecule and forming what’s called a peptide bond (chemically, an amide bond). String enough of these together and you have a peptide.
The terminology shifts with length:
The cutoff between “peptide” and “protein” is fuzzy. Insulin is 51 amino acids — most people call it a protein, but its small size makes it behave a lot like a peptide.
Your body makes peptides in two ways. First, it can build them from scratch using ribosomes — the cellular machines that read genes and assemble amino-acid chains. Second, it can take a larger protein and clip it into smaller active peptides; oxytocin and many hormones are made this way.
Scientists make them in the lab using a technique called solid-phase peptide synthesis — invented by Bruce Merrifield (Nobel Prize, 1984). One amino acid at a time gets snapped onto a growing chain anchored to a resin bead. Modern automated synthesizers can build a 30-amino-acid peptide in hours.
Peptides almost never enter cells directly. Instead, they bind to receptors on the cell surface — protein structures that recognize one specific peptide shape (think key/lock). When the right peptide binds, the receptor changes shape and triggers a cascade of events inside the cell: gene expression changes, calcium gets released, an enzyme gets activated, etc.
The specificity is part of why peptides are interesting therapeutics: a peptide drug usually hits one receptor type with high precision, instead of broadly affecting many systems the way small-molecule drugs often do.
Your digestive system is designed to break protein-like things down into their component amino acids — that’s how you extract nutrition from food. So when you swallow a peptide drug, the stomach’s acid and the peptidase enzymes in your gut treat it like dinner and rip it apart before it can be absorbed.
To avoid this, most peptide drugs are injected (usually subcutaneously — into fat just under the skin) or delivered through the nose. A few newer drugs use chemical tricks to survive the gut: oral semaglutide pairs the peptide with a permeation enhancer that lets it slip through the stomach lining quickly.
A peptide is a polymer of α-amino acids joined by amide (peptide) bonds — a condensation reaction between the carboxyl group (—COOH) of one residue and the α-amino group (—NH₂) of the next, releasing one molecule of water per bond. The α-carbon carries an amino group, a carboxyl group, a hydrogen, and a side chain (R group). The 20 proteinogenic amino acids encode the side chains that determine charge, polarity, aromaticity, and steric bulk.
The peptide-vs-protein cutoff is conventional rather than chemical — typically drawn around 50 residues, but molecules in the 30–100 range get classified either way depending on context (insulin’s 51 residues are usually called a protein; somatostatin’s 14 residues, a peptide).
Endogenous peptides are produced either ribosomally (translated from mRNA) or non-ribosomally (by specialized enzyme complexes — common in microbes, rare in mammals). Most signaling peptides are translated as longer prepropeptides that get cleaved by signal peptidases and prohormone convertases into the mature active form. Oxytocin, for example, is excised from a 125-residue precursor that also yields neurophysin I.
Synthetic production overwhelmingly uses solid-phase peptide synthesis (SPPS), originally developed by Bruce Merrifield. The growing chain is anchored to a resin via the C-terminus. Each cycle: deprotect the α-amino group, couple the next protected amino acid, wash off excess reagent. Fmoc protection chemistry dominates modern practice over the older Boc strategy. Cleavage from the resin yields the crude peptide, which is then purified by reversed-phase HPLC and identity-confirmed by mass spectrometry. Manufacturing quality assurance (whether a vial actually contains what the label says) is the central sourcing concern for any non-pharmaceutical peptide.
Most therapeutic peptides act on cell-surface receptors — they’re too hydrophilic to cross lipid membranes passively. Major target classes:
Affinity is typically nanomolar (binding constant 10⁻⁹ M range), and receptor selectivity is much higher than is usually achievable with small-molecule drugs targeting the same biology — a major reason peptides are favored for targets like incretin receptors where off-target activity carries meaningful toxicity.
Native peptides are pharmacokinetic nightmares:
Modern peptide drug design exists largely to defeat the PK problems above. Common modifications:
Peptides sit in a sweet spot between two more established drug classes. Small molecules have excellent pharmacokinetics and cheap manufacturing but struggle to selectively engage large, complex receptor surfaces (the GLP-1 receptor binding pocket, for example, is fundamentally a peptide-shaped slot — small molecules can fit there only with considerable engineering compromise). Monoclonal antibodies give exquisite target selectivity but are large, expensive, must be refrigerated, and don’t cross most tissue barriers.
Peptides bridge the gap: synthetic enough that production is tractable at scale, structured enough to engage protein-protein interfaces antibodies handle well, and (with the engineering tricks above) durable enough to dose weekly. The commercial success of the GLP-1 receptor agonist class has pulled massive industry investment into the modality; expect the catalog of approved and investigational peptide drugs to keep growing through this decade.
What each grade means, what kinds of studies it requires, and why a grade alone is not a recommendation.
FDA-approved, compounding pharmacy, gray market, research-only — what each path actually looks like.
The standard reconstitution medium for injectable peptides — and why it’s not interchangeable with sterile water.
Search and filter the full catalog by goal, route, evidence grade, and more.