Peptides DB

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What Are Peptides? A Definition for Researchers and Patients

A clear definition of peptides — what makes a peptide a peptide, how they differ from proteins and small molecules, why they're injected, and the major classes used in research and medicine.

Published Jun 14, 20265 min read

A peptide is a short chain of amino acids — typically between 2 and 50 — linked by peptide bonds. Peptides sit between single amino acids and full proteins in size. They are the signaling currency of the body: hormones like insulin, oxytocin, and glucagon are peptides; so are the research compounds discussed throughout this site. This article defines the term, explains why peptides are usually injected rather than swallowed, and surveys the major peptide classes you'll encounter.

The chemistry in one paragraph

Amino acids are the building blocks of proteins. There are 20 standard ones used by the human body. A peptide is formed when amino acids link via peptide bonds — an amide bond between the carboxyl group of one amino acid and the amino group of the next, releasing a water molecule. Two linked amino acids form a dipeptide; three a tripeptide; many an oligopeptide or polypeptide; "many" turning into a folded structure that we typically call a protein. The cutoff between "peptide" and "protein" is fuzzy and conventional — most usage puts it around 50 amino acids.

How peptides differ from proteins, hormones, and small-molecule drugs

Small molecule Peptide Protein
Size < 1 kDa typically 0.2–6 kDa 6 kDa to many MDa
Amino acids None (most) 2–~50 50+
Oral bioavailability Often good Usually poor (digested) Almost zero (digested)
Production Chemical synthesis Chemical synthesis or recombinant expression Recombinant expression
Target specificity Variable High High
Examples Aspirin, caffeine Insulin, BPC-157, Semaglutide Antibodies, growth factors

"Hormone" is a functional category, not a structural one. A peptide hormone is a hormone that happens to be a peptide (insulin, oxytocin, ACTH, GLP-1). A steroid hormone (testosterone, cortisol) is a small molecule. A thyroid hormone is also a small molecule. "Peptide" describes the structure; "hormone" describes the role.

Why peptides are usually injected

The digestive system is built to break peptides down. Stomach acid denatures them; proteases in the small intestine cleave them into individual amino acids; what little intact peptide reaches the gut wall mostly doesn't cross. Oral peptide doses end up as amino-acid nutrition rather than signaling molecules.

Three workarounds exist:

  1. Subcutaneous or intramuscular injection — the default. Bypasses digestion entirely. Subcutaneous is preferred for the peptides in this library because absorption is slower and steadier. See how to inject peptides.
  2. Special oral formulations — protective coatings, absorption-enhancers, or hybrid small-molecule structures. Orforglipron is a small molecule that targets the GLP-1 receptor without being a peptide; Rybelsus is oral Semaglutide with a delivery agent that boosts gastric absorption.
  3. Intranasal or sublingual — works for some short peptides (Selank, Semax, PT-141). Bypasses digestion but bioavailability is variable.

For research-use peptides, subcutaneous injection is almost always the route. See how to reconstitute peptides and where to inject peptides for the operational guides.

Major peptide classes

A non-exhaustive overview of the classes you'll encounter on this site. Each has its own category hub with the full per-peptide list.

Metabolic and weight-loss peptides

GLP-1, GIP, glucagon, and amylin receptor agonists. The fastest-growing class in modern medicine. Includes Semaglutide, Tirzepatide, Liraglutide, Retatrutide, Cagrilintide, Survodutide, Mazdutide, Orforglipron, Tesamorelin, AOD-9604. See peptides for weight loss.

Growth hormone / IGF-1 axis

Either stimulate the body's own GH release (GH secretagogues: Sermorelin, CJC-1295, Ipamorelin, MK-677, Hexarelin, GHRP-2, GHRP-6) or supply GH or IGF-1 directly (HGH/Somatropin, IGF-1 LR3, IGF-1 DES). Used for growth-deficiency therapy and in research for body-composition and recovery endpoints. Hub.

Healing / regenerative peptides

BPC-157, TB-500 (Thymosin Beta-4 fragment), GHK-Cu, Cartalax, Pinealon, MGF. Used in research for soft-tissue repair, wound healing, joint and tendon recovery. Hub.

Cognitive / nootropic peptides

Semax, Selank, Cerebrolysin, Dihexa, P21, NA-Semax Amidate. Russian-origin nootropic peptides (Semax, Selank, etc.) plus newer Western-developed synaptic-plasticity peptides (Dihexa, P21). Hub.

Immune / inflammation peptides

KPV, Thymosin Alpha-1, LL-37, Thymulin, Thymalin, VIP. Immune-modulating and anti-microbial peptides; several have decades of clinical research behind them. Hub.

Sexual-health peptides

PT-141 (Bremelanotide), Kisspeptin, HCG, Gonadorelin, HMG, Oxytocin, Melanotan II. Affect reproductive hormone axis, libido, sexual function. Hub.

Skin / cosmetic peptides

GHK-Cu, AHK-Cu, Argireline, Matrixyl, SNAP-8, Melanotan I/II. Topical use is mainstream cosmetic chemistry; injected use is a research-only frontier. Hub.

Longevity / mitochondrial peptides

Epitalon, NAD+ (technically a coenzyme, not a peptide), SS-31 (Elamipretide), FOXO4-DRI, MOTS-c, Humanin. Most research-stage; mechanism stories range from telomerase activation to mitochondrial cardiolipin protection. Hub.

Bioregulator peptides

Khavinson-school short peptides (Pinealon, Cortagen, Vesugen, Livagen, Pancragen, etc.). Russian gerontological research tradition; modern Western trials limited. Hub.

Are peptides drugs?

Some are FDA-approved drugs (insulin, Semaglutide, Tirzepatide, HCG, Tesamorelin, HGH, Teriparatide, PT-141). These are prescription medications with full pharmacovigilance.

Many are sold legally only as research chemicals, labeled "for research purposes only," and are not approved for human use. The product-quality and regulatory situation is more nuanced — see are peptides safe?.

How peptides are made

Two main routes:

  1. Solid-phase peptide synthesis (SPPS) — amino acids are linked one at a time on a solid resin in a chemical reactor. Most short research peptides are made this way.
  2. Recombinant expression — engineered bacteria, yeast, or mammalian cells produce the peptide as a normal byproduct of their own protein synthesis. Insulin, HGH, and most longer peptides are made this way.

The production method doesn't affect the final peptide — a peptide is a peptide regardless of origin. It does affect manufacturing-quality risk: contamination patterns differ between synthesis routes.

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