Peptides are short chains of amino acids. Amino acids are the same building blocks that make up proteins. Your body makes thousands of peptides, and they run some of its most important jobs. Insulin, oxytocin and the appetite hormone GLP-1 are all peptides.
Peptides also show up in pharmacies, skincare aisles and, more and more, social media feeds. This guide explains what peptides are, what they do in the body, and how to tell a proven peptide drug from an online trend.
The short definition
A peptide is two or more amino acids linked in a chain. The link between each pair is a chemical bond called a peptide bond. That bond gives the molecule its name.
Scientists use a few labels based on chain length:
- Dipeptide: two amino acids.
- Oligopeptide: a short chain, often under 20 amino acids.
- Polypeptide: a longer chain.
- Protein: a long chain, usually folded into a complex 3D shape.
No single cutoff separates a peptide from a protein. For drug rules, the FDA draws the line at 40 amino acids. A chain of more than 40 counts as a protein. Our guide to peptides vs proteins vs amino acids explains why that line matters.
How the body makes peptides
Cells build peptides from instructions in your DNA. Many start as a larger precursor protein. Enzymes then cut the precursor into one or more active peptides. Insulin works this way. Cells in the pancreas make a precursor called proinsulin, then trim it into active insulin.
The body also breaks peptides down fast. Enzymes in the blood and gut chop most of them apart within minutes. That short life helps the body switch signals on and off with precision. It also explains why many peptide drugs need special design to last longer.
What peptides do in the body
Peptides carry messages and defend the body. Here are the main jobs.
Hormones
Many hormones are peptides. They travel through the blood and attach to receptors on target cells. Insulin lowers blood sugar. Glucagon raises it. GLP-1 boosts insulin release after meals and signals fullness. See our guide to peptide hormones in the body.
Brain and nerve signals
Some peptides act as messengers in the nervous system. Endorphins, for example, help control pain.
Defense
Antimicrobial peptides help kill bacteria and other germs. Skin, gut lining and white blood cells all make them.
Structure and repair
Peptide fragments from larger proteins, such as collagen, take part in tissue repair signals. Research on many of these fragments is still early.
Peptides as medicines
Doctors have used peptide drugs for over a century. Insulin therapy began in the 1920s and changed type 1 diabetes from a fatal disease into a manageable one.
Today the FDA has approved dozens of peptide drugs. They treat diabetes, obesity, osteoporosis, some cancers, HIV and rare hormone disorders. Semaglutide (Ozempic, Wegovy) and tirzepatide (Mounjaro, Zepbound) are the best-known current examples. See our list of FDA-approved peptide drugs.
Most peptide drugs come as injections. Stomach acid and gut enzymes break peptides apart before the body absorbs them. A few tablets exist, such as oral semaglutide, but they need special ingredients to survive digestion.
Peptides in skincare and supplements
Two other product groups use the word “peptide”:
- Skincare: creams and serums with peptides such as copper peptides (GHK-Cu). These are cosmetics. The FDA does not approve cosmetics before sale.
- Supplements: collagen peptides are broken-down pieces of collagen protein. They are dietary supplements, not drugs.
Both groups face lighter rules than drugs. Makers do not need to prove a benefit before they sell.
Peptides sold online
A large online market sells peptides such as BPC-157 and TB-500. Most of these have no FDA approval for any use in people. Many carry a “research use only” label. That label means no one checked the product for purity, dose or sterility for human use. Read more about what “research use only” means.
Most claims about these peptides come from studies in rats and mice. Results in animals often fail to hold up in people.
How to judge a peptide claim
Ask three questions every time you see a claim about a peptide:
- Who was studied? Cells in a dish, animals or people?
- How strong was the study? A randomized controlled trial with a comparison group carries far more weight than a small uncontrolled study.
- Did the FDA review it? Approval means the FDA reviewed trial data for a specific use.
Every page in our Peptide Library answers these questions with an evidence scale from lab studies up to FDA approval.
Key takeaways
- Peptides are short chains of amino acids. The body makes thousands of them.
- Many hormones, including insulin and GLP-1, are peptides.
- Dozens of peptide drugs have FDA approval for specific uses.
- Many peptides sold online have no approval and little human data.
- Always check who was studied and whether the FDA reviewed the evidence.