What are peptides?

Amino acids are the building blocks of both peptides and proteins. When the acid group of one amino acid links to the amino group of the next, the covalent link formed is called a peptide bond, and a short chain of these is a peptide. [1][2]

The National Human Genome Research Institute describes a peptide as a short chain of typically 2 to 50 amino acids, with longer chains of 51 or more usually called polypeptides. [2]

These words describe different things, and they are often used loosely.

Size terms. A peptide is short. A polypeptide is a longer chain. A protein is a large polypeptide, long and complex enough to fold into a defined three dimensional shape. [1][2][3] The exact boundary between a peptide and a protein is a convention rather than a hard rule, which is why different sources quote different cutoffs, commonly somewhere between 50 and 100 amino acids. [3]

Function terms. A hormone is defined by what it does, not its size. A hormone is a signal released by one tissue that travels to target cells and triggers a response. [1][4] A molecule can be both a size and a function at once. Insulin is a protein that acts as a hormone. GLP-1 is a peptide that acts as a hormone. [4]

Many active peptides work by binding to receptors on the surface of cells. Binding changes the shape of the receptor, which switches signalling pathways inside the cell on or off. [1][4]

Through this mechanism, peptides act as hormones, as messengers in the nervous system, and as part of the immune response. [1] An everyday example is the family of natriuretic peptides released by the heart, which help regulate blood pressure and fluid balance. [1]

The compounds people discuss fall into a few broad groups. The examples below are descriptive only.

Metabolic and incretin peptides act on the incretin system, which links the gut to insulin release. GLP-1 is an incretin hormone that stimulates glucose dependent insulin secretion. [5][6] Examples studied or approved in this area include semaglutide, a GLP-1 receptor analogue, and tirzepatide, which acts on both the GIP and GLP-1 receptors. [7][9]

Tissue repair peptides are studied, largely in animal models, for wound healing and soft tissue recovery. Examples named in the literature include BPC-157, a synthetic chain of fifteen amino acids, and TB-500, a fragment of the protein thymosin beta-4. [10][11] Neither is an approved medicine for human use. [11]

Growth hormone secretagogues are peptides that prompt the body to release its own growth hormone. Examples include ipamorelin, described in the literature as a selective growth hormone secretagogue, and CJC-1295, an analogue of growth hormone releasing hormone. [12][13]

Cosmetic peptides are applied to the skin and are intended to influence processes such as collagen production. One studied example is palmitoyl pentapeptide-4. [16]

This is the distinction that matters most and that many sources skip.

Some peptides are approved medicines, manufactured to pharmaceutical standards and reviewed by regulators. Semaglutide, for example, was approved by the FDA in 2017. [7][8] In the United States, medicines require FDA approval, and in the European Union they require a marketing authorisation before they can be sold as medicines. [8][17]

Other peptides are sold as research grade or labelled for laboratory use only. That labelling is not a sign of approval and is not evidence of safety or quality. In the United States, the FDA reviews which substances may be used in pharmacy compounding, and it placed several peptides, including BPC-157, into a category that compounders may not use while safety is evaluated. [14] A federal advisory committee meeting on several of these peptides was scheduled for July 2026. [15]

With research grade products, much of the documented harm comes from contamination or mislabelling rather than from the molecule itself.