What Are Peptides? A clinical primer on peptide therapy

Peptides are short chains of amino acids joined by peptide bonds. They are commonly defined as containing roughly 2 to 50 amino acids — long enough to carry specific biological information, short enough to be synthesized precisely. Longer chains that fold into complex three-dimensional structures are usually called proteins.

In the body, peptides act mainly as messengers. They carry instructions between cells, glands, and organs: signaling the pituitary to release growth hormone, telling the pancreas to release insulin, regulating appetite through the gut–brain axis, or modulating inflammation. The body makes thousands of them. Insulin, oxytocin, glucagon, and GLP-1 are all peptides.

Therapeutic peptides are usually synthetic versions or modified analogs of these natural signals, designed to reproduce, strengthen, or prolong a specific effect.

Most therapeutic peptides work by binding to a receptor on the surface of a cell — most often a G protein–coupled receptor. That binding triggers signaling cascades inside the cell (such as the cAMP pathway) that amplify the message into an effect: hormone secretion, changes in gene expression, cell migration, or tissue repair.

Receptor specificity is the defining feature. A peptide built to fit one receptor can act on the tissues that express that receptor with comparatively little activity elsewhere. It is also why peptides with similar names can behave very differently, and why the evidence for one peptide says nothing about another.

Some peptide therapies work upstream of a hormone rather than replacing it. Growth hormone secretagogues such as sermorelin and ipamorelin stimulate the pituitary to release its own growth hormone within normal feedback loops, instead of supplying growth hormone directly.

Most peptides are broken down by digestive enzymes, so oral bioavailability is generally poor. That is why the majority are given by subcutaneous injection, with nasal sprays, sublingual troches, and topicals used where a peptide's stability and the clinical goal allow.

Compounding lets a prescriber specify a strength, route, and formulation for an individual patient. A licensed 503A pharmacy prepares it under USP sterile-compounding standards and assigns a beyond-use date. Many injectable peptides are dispensed as freeze-dried powder that must be reconstituted before use — see how to reconstitute peptides and the peptide calculator.

This is also the clearest line between prescription peptides and the "research peptides" sold online. Research-use vials are labeled not for human use, carry no assurance of identity, purity, or sterility, and bypass the prescriber entirely.

An FDA-approved peptide drug has passed the full approval process for specific indications. A compounded peptide is regulated differently. Under Section 503A of the Food, Drug, and Cosmetic Act, a pharmacy may compound for an individual patient's prescription only from a bulk drug substance that has a USP monograph, is a component of an approved drug, or appears on FDA's 503A Bulks List.

Many wellness peptides have no monograph and are not part of an approved drug, so their status depends on the Bulks List process. At the July 2026 Pharmacy Compounding Advisory Committee meeting, FDA's briefing documents proposed not adding seven peptides — including BPC-157, TB-500, and KPV — to the list, while others such as sermorelin and ipamorelin remain under evaluation. Our FDA peptide status tracker follows each compound, and the 503A compounding pharmacy guide explains the framework.