What are peptides?
The plain-English version: peptides are short chains of amino acids, the same building blocks proteins are made of, but smaller. Which turns out to matter for a lot of reasons.
The basics
Amino acids are small molecules your body uses to build essentially every structural and functional protein. There are 20 standard amino acids, and they get linked together in specific sequences to form chains.
When a chain has only a handful of amino acids linked together, we call it a peptide. When the chain is long enough to fold into a complex three-dimensional structure — typically more than 50 amino acids — we call it a protein. The line between them is fuzzy. Insulin, for instance, is sometimes classified as a peptide (51 amino acids) and sometimes as a small protein.
Your body makes thousands of different peptides. Many act as signaling molecules — the chemical messages your cells use to tell each other what to do. Others are structural, anti-microbial, or enzymatic. Peptides aren't a foreign pharmaceutical category; they're a normal part of how biology works.
Where therapeutic peptides come from
Already in your body
Your pituitary, gut, pancreas, and immune system all produce signaling peptides. Insulin, oxytocin, glucagon, and GLP-1 are all examples of peptides your body makes naturally.
Modified versions
Most therapeutic peptides are analogs: small modifications of a natural peptide designed to be more stable, more selective, or longer-acting. Semaglutide and liraglutide are GLP-1 analogs. Sermorelin is a GHRH analog. The natural peptide's backbone stays; specific residues are changed.
Designed from scratch
A few therapeutic peptides were designed de novo by researchers — built from amino acid sequences that don't occur naturally but bind a target receptor. These are rarer, and their evidence bases vary widely.
Peptides you've already heard of
Peptide therapy isn't new. Some of the most widely used prescription medicines in the world are peptides.
Insulin (51 amino acids)
The first therapeutic peptide, isolated and purified in 1921 by Banting and Best. Tens of millions of people with diabetes use insulin daily. It's a peptide.
Oxytocin (9 amino acids)
A hormone involved in social bonding, childbirth, and lactation. Synthetic oxytocin is used medically to induce labor and manage postpartum hemorrhage.
GLP-1 agonists: semaglutide, liraglutide (30+ amino acids)
The category behind Ozempic, Wegovy, Mounjaro, and Zepbound — a handful of the most-prescribed medications in the U.S. right now. All peptide analogs of endogenous GLP-1.
ACTH / cosyntropin (24 amino acids)
Used diagnostically for decades to evaluate adrenal function. A peptide drug with a specific, well- characterized mechanism.
Why peptides matter therapeutically
Most small-molecule drugs bind their target by shape matching. That works, but small molecules often bind multiple targets — the intended one plus a handful of similar-looking receptors elsewhere in the body. The unintended binding is where side effects tend to come from.
Peptides can be much more selective. Because they're longer than small molecules, they engage their target through more specific interactions — think of it as a key with more ridges, which reduces the chance of opening unintended locks. For a lot of biological pathways, a well-designed peptide analog can be meaningfully more targeted than any small molecule would be.
That selectivity is the main reason peptides have become a growing focus for pharmaceutical research. Dozens of peptide drugs are now in development, and the pace has accelerated as synthesis and stability technologies have improved.
The limits
Peptides have real trade-offs that drive how they're dosed and delivered.
Oral bioavailability is usually poor. Peptides are made of the same stuff as dietary protein, and your digestive system is very good at breaking them down before they can reach the bloodstream intact. That's why most therapeutic peptides are given by injection. Exceptions exist (oral semaglutide, for instance) but they require special formulations and typically deliver a small fraction of the subcutaneous dose.
Half-lives are short without modification. Native GLP-1 is broken down within minutes. Semaglutide was engineered with specific modifications to extend its half-life to about seven days, which is why it can be dosed weekly rather than multiple times per day.
Cost of synthesis varies. Short peptides are relatively cheap to produce. Longer or more chemically complex peptides cost more. Compounding and commercial economics both reflect this.
Keep reading
How peptides work
Receptor binding, amplification cascades, and what makes peptide signaling different from small-molecule drugs.
The Peptide Library
Research-backed pages on each peptide currently available through PepScribe, plus research-only pages on compounds awaiting FDA reclassification.