What Are Peptides? Definition, Examples and How They Differ From Proteins

Peptides are short chains of amino acids, the same building blocks that make up proteins. They are behind some of the best-known medicines of the last century, from insulin to semaglutide, and behind the research vials sold online as "peptides". This page covers what a peptide is, where a peptide ends and a protein begins, real examples with their lengths, what peptides do in the body and why most are injected. For the chemistry of the link itself, see the peptide bond.

What is a peptide?

The international nomenclature committee of IUPAC and IUBMB defines it in one line: "A peptide is any compound produced by amide formation between a carboxyl group of one amino acid and an amino group of another. The amide bonds in peptides may be called peptide bonds" (IUPAC-IUB JCBN, 1983). Each time two amino acids join, the elements of water are removed, and what is left of each amino acid is called a residue. A biology textbook puts it the same way: "Peptide bond formation is a dehydration synthesis reaction", and "As more amino acids join to this growing chain, the resulting chain is a polypeptide" (OpenStax Biology 2e).

The amino acids come from a set of 20 that are "commonly found in proteins and are represented in the genetic code" (IUPAC-IUB, Table 1). The number of residues gives the name: a dipeptide has two, a tripeptide three, a hexapeptide six, and chemists may simply write "a 22-peptide". Peptides with fewer than about 10 to 20 residues may also be called oligopeptides, and longer ones polypeptides (IUPAC-IUB).

Peptide vs protein: where the line is

There is no single chemical cut-off. IUPAC says polypeptides "of specific sequence of more than about 50 residues are usually known as proteins, but authors differ greatly on where they start using this term" (IUPAC-IUB). The US FDA needed a hard line for the law, and since 23 March 2020 "the term protein means any alpha amino acid polymer with a specific, defined sequence that is greater than 40 amino acids in size" (Federal Register, 21 February 2020; 21 CFR 600.3). FDA's draft guidance on peptide drugs uses the other side of that line: a peptide is "any polymer composed of 40 or fewer amino acids" (FDA draft guidance, December 2023).

The FDA line matters because it decides which law applies. A peptide of 40 or fewer amino acids that meets the definition of a drug is regulated as a drug, including "the requirement that new drugs must be approved under section 505(c) of the FD&C Act before they can be marketed" (FDA draft guidance). Longer chains count as proteins and go through the biologics route. By the FDA's own example, insulin, with chains of 21 and 30 amino acids, "clearly is a 'protein'" (FDA rule), even though many papers still call it a peptide.

Examples of peptides, with their length

Semaglutide and tirzepatide are modified peptides. Semaglutide has 94% sequence homology to human GLP-1 and carries a fatty acid on lysine 26 that binds albumin, which its label names as the main reason it lasts (Wegovy label). Tirzepatide is based on the GIP sequence, contains an amino acid the genetic code does not use (Aib, at positions 2 and 13) and carries a C20 fatty diacid (Zepbound label; Coskun 2018). Not everything sold as a "peptide" is one: orforglipron, sold as Foundayo, is "a small-molecule, nonpeptide" GLP-1 drug (ATTAIN-1, NEJM). For a wider list with FDA status, see the list of peptides and what they do.

What peptides do in the body

FDA's draft guidance says peptides "often serve as signaling molecules for many physiological functions" (FDA). A few examples from the primary literature:

• Hormones. Oxytocin acts on a "typical class I G protein-coupled receptor" and drives uterine contraction in labour and milk ejection (Gimpl and Fahrenholz 2001). Semaglutide is built on the GLP-1 sequence and tirzepatide on the GIP sequence (Wegovy label; Zepbound label).
• Brain signals. The enkephalins, the body's own ligands for opiate receptors, are two pentapeptides, Tyr-Gly-Gly-Phe-Met and Tyr-Gly-Gly-Phe-Leu (Hughes 1975).
• Defence. Animals and plants make "potent, broad-spectrum antimicrobial peptides" against bacteria, fungi, viruses and protozoa (Zasloff 2002, Nature).
• Antioxidant. Glutathione is a tripeptide of glutamate, cysteine and glycine (Nature 2026).

Most peptides act on the outside of cells. A 2022 review found that more than 90% of peptides in clinical development hit extracellular targets such as G protein-coupled receptors, because "Peptides are generally unable to cross the cell membrane" (Wang 2022).

Why most peptides are injected

The body breaks peptides down fast. Enzymes that cut peptide bonds are everywhere, so "proteolytic degradation of many peptides is rapid and not limited to organs typically associated with drug elimination, such as the liver" (FDA draft guidance). In the gut, stomach acid and pepsin, trypsin and chymotrypsin "rapidly degrade the peptide chain", and "the oral bioavailability of most native peptides in adults is negligible" (Pharmaceutics 2026 review); a second 2026 review puts it below 1% for most compounds (Front Drug Deliv 2026). That is why "Protein and peptide therapeutics require parenteral administration", in other words injection, as a 2020 review put it (Drucker 2020, Nat Rev Drug Discov).

Oral semaglutide shows how hard the workaround is. The tablet carries an absorption aid, SNAC, and even so the label puts absolute bioavailability at about 0.4% to 1% for Rybelsus and 1% to 2% for the newer Ozempic tablets (FDA label, August 2026). More in our oral semaglutide guide.

A short history of peptides

• 1901. Emil Fischer, with Fourneau, made the first synthetic dipeptide, glycylglycine, and established "the type of bond that would connect them together in chains, namely, the peptide bond" (Nobel biography).
• 1921. Insulin, described by a 2022 review as "the first therapeutic peptide" (Wang 2022).
• 1953. Vincent du Vigneaud sequenced and synthesised oxytocin, "the first peptide hormone to have its sequence of amino acids determined"; Nobel Prize 1955 (Nobel Prize).
• 1955. Frederick Sanger, having determined the amino acid sequences of insulin's two chains, identified how they are linked; Nobel Prize 1958 (Nobel Prize).
• 1963. Bruce Merrifield published solid phase peptide synthesis, building chains on a solid support; Nobel Prize 1984 (Nobel Prize).
• 2021. "More than 80 peptide drugs have reached the market" (Muttenthaler 2021, Nat Rev Drug Discov).

What "research peptides" are

Online vendors sell synthetic peptides as powder in vials, labelled for laboratory research use only. Apart from copies of approved drugs, most of them, such as BPC-157 or TB-500, have no approval for human use anywhere. A peptide marketed as a drug without approval breaks the FD&C Act requirement quoted above, and the FDA sends warning letters to sellers over it (examples in our bacteriostatic water guide). Our guides cover the parts that matter if you study them:

• List of peptides and what they do, with FDA status for each.
• Are peptides safe? What the trial and case evidence shows.
• Research peptides legal status, with a page for every US state.
• How to reconstitute peptides and bacteriostatic water, for the diluent and the units maths.
• Peptide storage, powder and mixed.
• How to buy peptides safely and how we score vendors.

Frequently asked questions

What is a peptide in simple terms?

A peptide is a short chain of amino acids, the building blocks of proteins, linked by peptide bonds. IUPAC defines it as any compound made by amide formation between the carboxyl group of one amino acid and the amino group of another.

What is the difference between a peptide and a protein?

Length. IUPAC says chains of more than about 50 residues are usually called proteins, though authors differ. For US law the FDA set the line at 40: more than 40 amino acids is a protein, 40 or fewer is a peptide.

Is insulin a peptide or a protein?

Under the FDA definition insulin is a protein: it has two chains of 21 and 30 amino acids, 51 in total, above the 40 cut-off. Many papers still call it a peptide hormone.

What do peptides do in the body?

Most work as signals. Oxytocin and GLP-1 are hormones, the enkephalins act on opiate receptors in the brain, glutathione is an antioxidant and antimicrobial peptides help fight infection. More than 90% of peptides in clinical development act on targets outside cells.

Why are peptides injected instead of taken as pills?

Stomach acid and gut enzymes break peptide bonds quickly, and the oral bioavailability of most native peptides is below 1%. Even oral semaglutide with an absorption aid reaches only about 0.4% to 2%, according to its FDA label.

How many peptide drugs are approved?

More than 80 peptide drugs had reached the market by 2021, for diabetes, cancer, osteoporosis, multiple sclerosis, HIV and chronic pain, according to a Nature Reviews Drug Discovery review.

Sources (checked 8 November 2026)

1. IUPAC-IUB Joint Commission on Biochemical Nomenclature. Nomenclature and symbolism for amino acids and peptides, Recommendations 1983, 3AA-11 to 3AA-13. Checked 8 November 2026.
2. IUPAC-IUB JCBN 1983, 3AA-1 and Table 1: alpha-amino acids incorporated into protein under mRNA direction. Checked 8 November 2026.
3. OpenStax. Biology 2e, section 3.4 Proteins, 2018. Checked 8 November 2026.
4. FDA. Definition of the term "biological product", final rule. Federal Register 2020;85(35):10057-10063. Checked 8 November 2026.
5. 21 CFR 600.3(h)(6), definition of protein (eCFR). Checked 8 November 2026.
6. FDA CDER. Clinical pharmacology considerations for peptide drug products, draft guidance, December 2023. Checked 8 November 2026.
7. Nature 2026 (glutathione, a tripeptide of glutamate, cysteine and glycine). Checked 8 November 2026.
8. Hughes J et al. Identification of two related pentapeptides from the brain with potent opiate agonist activity. Nature 1975;258:577-580. Checked 8 November 2026.
9. Int J Mol Sci 2022 (oxytocin, a nine amino acid neuropeptide). Checked 8 November 2026.
10. Eur Heart J Suppl 2025 (GLP-1, a 30 or 31 amino acid peptide). Checked 8 November 2026.
11. Pharmaceuticals 2026 review (semaglutide, a modified 31 amino acid analog of GLP-1). Checked 8 November 2026.
12. Coskun T et al. LY3298176, a novel dual GIP and GLP-1 receptor agonist. Mol Metab 2018. Checked 8 November 2026.
13. Orforglipron, an oral small-molecule GLP-1 receptor agonist for obesity treatment (ATTAIN-1). N Engl J Med 2025;393(18):1796-1806. Checked 8 November 2026.
14. Wegovy (semaglutide) FDA label, DailyMed, June 2026. Checked 8 November 2026.
15. Zepbound (tirzepatide) FDA label, DailyMed, September 2026. Checked 8 November 2026.
16. UniProt P01215, glycoprotein hormones alpha chain. Checked 8 November 2026.
17. UniProt P0DN86, choriogonadotropin subunit beta 3. Checked 8 November 2026.
18. Gimpl G, Fahrenholz F. The oxytocin receptor system. Physiol Rev 2001;81:629-683. Checked 8 November 2026.
19. Zasloff M. Antimicrobial peptides of multicellular organisms. Nature 2002;415:389-395. Checked 8 November 2026.
20. Wang L et al. Therapeutic peptides: current applications and future directions. Signal Transduct Target Ther 2022. Checked 8 November 2026.
21. Pharmaceutics 2026 review (salcaprozate sodium and oral peptide delivery). Checked 8 November 2026.
22. Front Drug Deliv 2026 review (oral peptide bioavailability). Checked 8 November 2026.
23. Drucker DJ. Advances in oral peptide therapeutics. Nat Rev Drug Discov 2020;19:277-289. Checked 8 November 2026.
24. Rybelsus and Ozempic tablets, FDA label (DailyMed), section 12.3, August 2026. Checked 8 November 2026.
25. Nobel Prize. Emil Fischer, biographical. Checked 8 November 2026.
26. Nobel Prize in Chemistry 1955, Vincent du Vigneaud, facts. Checked 8 November 2026.
27. Nobel Prize in Chemistry 1958, Frederick Sanger, facts. Checked 8 November 2026.
28. Nobel Prize in Chemistry 1984, Bruce Merrifield, facts. Checked 8 November 2026.
29. Muttenthaler M et al. Trends in peptide drug discovery. Nat Rev Drug Discov 2021;20:309-325. Checked 8 November 2026.

This is a chemistry and regulation explainer; research peptides are for laboratory research use only. Labels and trial records change; the leaflet in your box and your prescriber come first. Educational content, not medical advice.