What Peptides Actually Are

A peptide is a short chain of amino acids — the same building blocks that make up every protein in the body — joined end to end by peptide bonds.

Length is the only thing separating a peptide from a protein. Chains of roughly two to fifty amino acids are called peptides. Longer than that and the same molecule gets called a protein. Nothing else changes.

Your body already runs on them. Insulin is a peptide. So is glucagon, oxytocin, and the growth hormone-releasing hormone your pituitary responds to. They are not foreign chemistry — they are the signalling language the body already speaks.

Peptides are not steroids

This is the most common misconception in the category, and the two are not related in any meaningful way.

Steroids are lipids. Every one of them — cortisol, testosterone, estrogen, and every anabolic-androgenic steroid — is built on a four-ring carbon skeleton derived from cholesterol. They are fat-soluble, which lets them pass straight through a cell membrane and bind receptors inside the cell, where they directly alter gene transcription.

Peptides are amino acid chains. They are generally water-soluble, generally cannot cross a cell membrane, and instead bind receptors on the cell surface, triggering a signalling cascade. They are broken down by the same protease enzymes that digest dietary protein.

	Peptides	Steroids
Built from	Amino acids	Cholesterol
Structure	A chain	Four fused rings
Solubility	Water-soluble	Fat-soluble
Receptor site	Cell surface	Inside the cell
Mechanism	Signalling cascade	Direct gene transcription
Broken down by	Protease enzymes	Hepatic metabolism

Different raw material, different shape, different target, different mechanism. The word “peptide” describes a chemical structure, not a category of effect — which is why the classes below are grouped by what these molecules are, not by what anyone claims they do.

Sequence-derived fragments

Short sequences copied directly out of a larger protein that already occurs in nature. Rather than synthesising something novel, these reproduce a specific active region of a known molecule.

BPC-157 is a fifteen-amino-acid sequence derived from a protein identified in gastric juice. TB-500 corresponds to a region of thymosin β4. AOD-9604 reproduces residues 176–191 of human growth hormone. KPV is the three-amino-acid tail of α-MSH.

The defining trait of the class is provenance: every one of them is a piece of something the body already produces.

Growth hormone secretagogues

Peptides that act upstream of growth hormone rather than replacing it. They bind receptors on the pituitary that govern release, which is a categorically different thing from administering the hormone itself.

Two mechanisms sit in this class. GHRH analogues — Tesamorelin, CJC-1295 — mimic growth hormone-releasing hormone. Ghrelin receptor agonists such as Ipamorelin bind GHS-R, a separate receptor that also influences release.

Grouped by receptor target, not by outcome.

Growth factors

Signalling proteins that regulate cellular growth and differentiation, and the modified analogues built from them.

IGF-1 LR3 is insulin-like growth factor 1 with a thirteen-amino-acid N-terminal extension and an arginine substituted at position three. Both modifications reduce its binding to IGF-binding proteins, which changes its behaviour in solution — the reason the modified form exists as a distinct research compound.

Incretin receptor agonists

Peptides that mimic incretins — the gut hormones released in response to food intake that signal to the pancreas and brain.

Retatrutide is a triple agonist, binding GLP-1, GIP and glucagon receptors. Tirzepatide is a dual agonist at GLP-1 and GIP. The generation before them targeted GLP-1 alone. The class is defined by which receptors a molecule engages and how many at once.

This is the most actively studied peptide class in current clinical research.

Melanocortin receptor peptides

Analogues of α-melanocyte-stimulating hormone, binding the melanocortin receptor family — MC1R through MC5R.

Melanotan II is a cyclic synthetic analogue that binds across several of these receptors rather than selecting one. The melanocortin system is widely distributed, which is why this receptor family attracts research interest well beyond pigmentation.

Metal-binding peptides

A small class defined by a structural property: the ability to chelate a metal ion as part of the functional molecule.

GHK-Cu is glycyl-L-histidyl-L-lysine bound to copper(II). The copper is not an additive — the tripeptide has a high natural affinity for it, and the complex is what occurs in human plasma. The metal is part of what the molecule is.

Mitochondrial-derived peptides

The newest class here, and the most structurally unusual. These are encoded not in nuclear DNA but within the mitochondrial genome itself.

MOTS-c is a sixteen-amino-acid peptide encoded in the mitochondrial 12S rRNA region. Its existence was only confirmed in the last decade, and the class it belongs to is still being mapped.

Neuropeptides

Peptides that act as signalling molecules within the nervous system.

Semax is a synthetic analogue of ACTH fragment 4–10, extended with a Pro-Gly-Pro tail that slows enzymatic breakdown. Selank is built the same way from tuftsin, an immunomodulatory peptide. Both were developed in Russia, where this class has a longer research history than in North America.

Regulatory peptides

Short sequences associated with endocrine regulation, typically very small.

Epitalon is a synthetic tetrapeptide — alanine, glutamic acid, aspartic acid, glycine — modelled on epithalamin, a pineal gland extract. At four amino acids it is among the smallest compounds in the category.

Blends

Multiple peptides supplied in a single vial at a fixed ratio. A blend is a formulation format rather than a class of its own — each component belongs to its own class above.

Why the classes matter

Grouping by structure and receptor target rather than by claimed effect is the honest way to organise this category. It reflects how the research literature itself is organised, and it makes the differences between compounds visible rather than blurring them into marketing language.

Every compound listed on this site is supplied for laboratory research use only. Nothing on this page describes an intended use in humans, and nothing here should be read as guidance for one.

For research use only — not for human consumption.