Peptides are short chains of amino acids that cells use to send instructions to one another. For a century, scientists have been learning to read those signals, copy them and improve them. This guide explains what peptides are made of, how they work, how they are made, and every way they are delivered into the body.
Every peptide is built from amino acids, the same 20 building blocks that make up every protein in your body. Think of them as an alphabet. A peptide is a short word spelled with those letters, and changing even one letter can change its meaning completely.
Amino acids are small molecules made of carbon, hydrogen, oxygen and nitrogen (two also contain sulfur). Each has the same backbone plus a unique side chain that makes it oily, water-loving, positively or negatively charged.
Peptide bonds link one amino acid's backbone to the next, head to tail, forming a chain. Your body makes these bonds constantly when it builds proteins from food.
Peptide or protein? It is a matter of length. U.S. regulators draw the line at 40 amino acids: a chain longer than that is legally treated as a protein3. Peptides are small enough to be made chemically, and large enough to carry very specific instructions.
Small modifications are common in modern peptides: a capped end, a mirror-image amino acid, a ring-shaped (cyclic) structure, or an attached fatty acid. These tweaks help a peptide survive longer or bind more tightly, and you will see them throughout this reference.
Most peptides never enter a cell. They travel to the cell's surface and fit into a receptor, a protein shaped to recognize one particular signal. When the right peptide clicks into place, the receptor passes a message inside, and the cell responds.
Not every peptide works this way. Here are the main patterns you will see across this reference:
Mimic a natural hormone so its receptor fires. This is called an agonist.
Signal a gland to release more of the body's own hormone, rather than supplying the hormone itself.
One engineered molecule designed to activate two or three related receptors at once.
Capture another signaling molecule so it cannot reach its receptor.
A few small peptides cross into cells and act on structures such as mitochondria.
Natural peptide signals are meant to be brief. Enzymes in the blood chop them apart and the kidneys filter them out, often within minutes, so the body can switch a signal off as quickly as it switched it on. That is useful for the body, but a challenge for medicine. Much of modern peptide science is about making signals last.
How scientists make peptides last longer:
A peptide only works if it reaches its receptor intact. The digestive system is designed to break proteins and peptides down into single amino acids for food, so swallowing a peptide usually destroys it. That one fact explains why most peptide medicines are injected, and why scientists keep inventing new ways around it.
A thin needle places the peptide in the fatty layer just under the skin. From there it seeps into tiny blood and lymph vessels and enters the circulation gradually, bypassing the digestive system entirely. Pen injectors and autoinjectors have made this route far simpler than it once was.
Delivered straight into a vein, so the full dose enters the blood at once and can be adjusted minute by minute.
Injected deeper, into muscle, which has a rich blood supply. Often used for larger hormone products.
For decades considered nearly impossible. Today a few peptides are approved as pills, using absorption enhancers that briefly help the peptide cross the stomach lining.
The lining of the nose is thin and full of blood vessels, letting some small peptides absorb without a needle. Researchers also study whether a fraction can travel toward the brain.
Creams and serums apply peptides to the skin's surface. The outer layer of skin is an excellent barrier, so topical peptides mostly act locally, in the upper layers, rather than throughout the body.
The lungs have an enormous surface area with a very thin barrier to the blood, which makes them an appealing route for some peptides.
A small rod placed under the skin by a clinician releases the peptide slowly over weeks or months.
A note on routes: research on one route does not automatically carry over to another. A peptide that works in a cream, or when injected in a rat, may behave very differently swallowed or sprayed. Each entry in this reference notes which routes were actually studied.
Early peptide medicines were extracted from animal glands. Today almost all are made in one of two ways: assembled chemically, one amino acid at a time, or produced by engineered cells.
Chemical synthesis. In the 1960s, chemist Bruce Merrifield invented solid-phase peptide synthesis5, work that earned him the 1984 Nobel Prize in Chemistry6. The idea is elegant: anchor the first amino acid to a tiny resin bead, then add the next one, wash away the leftovers, and repeat. Because the growing chain stays attached to the bead, the process can be automated, and machines now build peptides dozens of amino acids long. Most of the peptides in this reference are made this way.
Recombinant production. Longer chains, such as insulin and growth hormone, are usually made by inserting the human gene into bacteria or yeast, which then produce the protein the same way your own cells would. Human insulin made this way was approved in 1982, replacing insulin taken from animals.
From raw peptide to finished vial:
Many peptides discussed here exist in two very different worlds: as approved medicines, and as research chemicals sold for laboratory use. Understanding the difference is one of the most important things a reader can take from this site.
| FDA-approved medicine | Research-grade compound | |
|---|---|---|
| Manufacturing | Made under current Good Manufacturing Practice (cGMP), with FDA inspections of the facility. | Made for laboratory use; not produced or inspected as a medicine for people. |
| Sterility | Every injectable lot is tested for sterility and bacterial endotoxins. | Not guaranteed. Sterility depends entirely on the producer. |
| Identity and purity | Verified to an official specification before each lot is released. | Documented by certificates of analysis that vary in quality and independence. |
| Evidence | Proven in controlled trials for a specific use. | Ranges from human trials to cell studies only. See each entry's evidence grade. |
| Labeling | Dosing, warnings and interactions set by regulators and guided by a clinician. | Labeled for research use only, not for human use. |
How to use this site well: start with a compound's evidence grade, read what the human studies actually found, and check the safety and open-questions sections. Any decision about your own health belongs in a conversation with a qualified clinician.
Peptide medicine is not new. What is new is the pace. In the last two decades, scientists have learned to design peptides that last for days, hit several receptors at once, and even survive as pills.
Insulin, a peptide hormone, is given to a person with diabetes for the first time, opening the era of peptide medicine1.
Vincent du Vigneaud synthesizes oxytocin, the first peptide hormone made in a laboratory, later honored with the 1955 Nobel Prize7.
Merrifield publishes solid-phase peptide synthesis, the method still used to make most peptides today5.
Human insulin made by engineered bacteria is approved, the first medicine of its kind.
Exenatide, based on a peptide from Gila monster venom, becomes the first approved GLP-1 drug.
Weekly semaglutide, then tirzepatide, then oral semaglutide reshape the treatment of diabetes and obesity. See the metabolic section.
Figures from a 2018 industry review1; both numbers have grown since. For a wider view of the field, see the 2021 review in Nature Reviews Drug Discovery2.
Browse all compounds, filter by evidence grade, and read the science behind each one.
Half-life and route examples are drawn from the FDA labels and studies cited in each compound's entry on this site.