Glossary of Peptide Terms

A comprehensive reference of key terms used throughout Peptides Know How and in the broader peptide therapeutics literature. Click any letter to jump to that section.

A

Agonist

A substance that binds to a receptor and activates it, producing a biological response that mimics the effect of the receptor's natural ligand. Many peptide drugs are receptor agonists -- for example, semaglutide is a GLP-1 receptor agonist that mimics the natural incretin hormone GLP-1. The opposite of an agonist is an antagonist, which blocks receptor activation. See also: Receptor.

Amino Acid

An organic molecule containing an amino group (-NH2), a carboxyl group (-COOH), and a variable side chain. Amino acids are the building blocks of peptides and proteins. There are 20 standard amino acids encoded by human DNA (e.g., glycine, alanine, leucine, tryptophan). Non-standard amino acids are sometimes incorporated into synthetic peptides to improve stability or activity. See also: Peptide Bond, Peptide.

Angiogenesis

The formation of new blood vessels from pre-existing vasculature. Angiogenesis is a critical process in wound healing, tissue repair, and embryonic development. Some peptides, including BPC-157, are studied for their pro-angiogenic properties, which may accelerate tissue repair by improving blood supply to injured areas. Pathological angiogenesis is also a hallmark of cancer progression.

B

Bacteriostatic Water

Sterile water for injection that contains 0.9% benzyl alcohol as a preservative. The benzyl alcohol inhibits microbial growth, allowing the vial to be punctured multiple times over 28 days. Bacteriostatic water is the standard solvent for reconstituting lyophilized peptides. It should not be used in neonates due to the risk of benzyl alcohol toxicity. See also: Reconstitution, Lyophilization.

Bioavailability

The fraction of an administered dose of a drug that reaches the systemic circulation in its active form. Intravenous injection has 100% bioavailability by definition. Subcutaneous injection of peptides typically has bioavailability in the range of 50-80%, depending on the peptide's molecular weight and formulation. Oral bioavailability of peptides is generally very low (<1-2%) because digestive enzymes degrade the peptide bonds before absorption. See also: Pharmacokinetics, Subcutaneous.

BPC (Body Protection Compound)

A partial sequence of a protein found in human gastric juice. The most studied form is BPC-157, a 15-amino-acid synthetic peptide with a molecular weight of 1419.53 Da. BPC-157 has been investigated in animal models for its effects on angiogenesis, tendon healing, and gastrointestinal protection, but it has not been tested in human clinical trials and is not FDA-approved. See also: Angiogenesis.

C

CAS Number

A unique numerical identifier assigned to every chemical substance by the Chemical Abstracts Service (CAS), a division of the American Chemical Society. CAS numbers are used to unambiguously identify chemicals regardless of naming conventions. For example, the CAS number for BPC-157 (acetate salt) is 137525-51-0. When evaluating a Certificate of Analysis, verify that the CAS number matches the expected peptide.

Certificate of Analysis (COA)

A document issued by a manufacturer or testing laboratory that reports the results of quality-control testing on a specific batch of a product. For peptides, a COA typically includes identity confirmation (mass spectrometry), purity (HPLC), sterility testing, and endotoxin testing. See our full guide: How to Read a Certificate of Analysis. See also: HPLC, Mass Spectrometry, Endotoxin.

Compounding Pharmacy

A licensed pharmacy that prepares custom medications for individual patients based on a prescriber's order. In the U.S., compounding pharmacies operate under Section 503A (traditional compounding) or Section 503B (outsourcing facilities) of the Federal Food, Drug, and Cosmetic Act. Compounding pharmacies can legally prepare peptide formulations not commercially available, subject to specific regulatory conditions. See our legal landscape guide for details.

Contraindication

A specific situation or condition in which a drug, procedure, or surgery should not be used because it may be harmful to the patient. Contraindications may be absolute (must never be used) or relative (may be used with caution if the benefit outweighs the risk). For example, semaglutide is contraindicated in patients with a personal or family history of medullary thyroid carcinoma.

D

DAC (Drug Affinity Complex)

A chemical modification that enables a peptide to bind to serum albumin in the bloodstream, dramatically extending its half-life. The most well-known example is CJC-1295 DAC, a growth hormone-releasing hormone (GHRH) analog where the DAC modification extends the half-life from approximately 30 minutes (without DAC) to approximately 6-8 days. This allows for less frequent dosing. See also: Half-Life, GHRH.

Dose-Response

The relationship between the dose of a drug administered and the magnitude of its effect. Dose-response curves are fundamental to pharmacology and are used to determine effective doses, maximum efficacy, and the therapeutic window. In peptide therapeutics, dose-response relationships can be complex due to receptor desensitization (tachyphylaxis), where increasing doses yield diminishing returns or adverse effects. See also: Titration.

E

Endotoxin

A component of the outer membrane of gram-negative bacteria (specifically, lipopolysaccharide or LPS). Endotoxins can cause fever, inflammation, and in severe cases, septic shock when introduced into the bloodstream. Even after bacteria are killed by sterilization, endotoxins can persist in solution. The standard test for endotoxin contamination is the Limulus Amebocyte Lysate (LAL) test, which uses horseshoe crab blood cell lysate. Injectable peptides should have endotoxin levels below 5 EU/kg body weight/hour. See our COA guide for more details.

F

FDA (Food and Drug Administration)

The U.S. federal agency responsible for regulating and supervising food safety, pharmaceuticals, medical devices, biologics, and cosmetics. The FDA approves new drugs (including peptide therapeutics) through the New Drug Application (NDA) or Biologics License Application (BLA) process after review of clinical trial data demonstrating safety and efficacy. The FDA also oversees compounding pharmacies and can take enforcement action against sellers of unapproved drugs. See our legal landscape guide.

G

GH (Growth Hormone)

A 191-amino-acid protein (also called somatotropin) produced by the anterior pituitary gland. GH stimulates growth, cell reproduction, and regeneration. Its release is regulated by GHRH (stimulatory) and somatostatin (inhibitory) from the hypothalamus. Synthetic GH (recombinant human growth hormone, rhGH) is FDA-approved for specific conditions including growth hormone deficiency, Turner syndrome, and HIV-associated wasting. Many peptide therapeutics (CJC-1295, ipamorelin, GHRP-6) work by stimulating the body's own GH secretion rather than replacing it directly.

GHRH (Growth Hormone-Releasing Hormone)

A 44-amino-acid peptide hormone produced by the hypothalamus that stimulates the anterior pituitary to synthesize and secrete growth hormone. Synthetic analogs of GHRH include sermorelin (the first 29 amino acids of GHRH), tesamorelin (FDA-approved for HIV-associated lipodystrophy), and CJC-1295 (a modified GHRH analog with extended half-life). See also: Secretagogue.

GIP (Glucose-Dependent Insulinotropic Polypeptide)

A 42-amino-acid incretin hormone produced by K-cells in the duodenum and jejunum. GIP stimulates insulin secretion in a glucose-dependent manner and has effects on fat metabolism and bone formation. Tirzepatide is a dual GIP/GLP-1 receptor agonist, making it the first approved drug to target both incretin receptors simultaneously. See also: GLP-1, Incretin.

GLP-1 (Glucagon-Like Peptide-1)

A 30-amino-acid incretin hormone produced by L-cells in the small intestine after food intake. GLP-1 stimulates insulin secretion, suppresses glucagon secretion, slows gastric emptying, and promotes satiety by acting on brain appetite centers. GLP-1 receptor agonists including semaglutide and tirzepatide are FDA-approved for type 2 diabetes and chronic weight management, representing the most commercially significant class of peptide drugs. See also: Incretin, GIP.

H

Half-Life

The time required for the plasma concentration of a drug to decrease by 50%. Half-life determines dosing frequency: drugs with short half-lives must be administered more frequently. Natural peptides often have very short half-lives (minutes) because they are rapidly degraded by peptidases. Pharmaceutical modifications such as fatty acid acylation (semaglutide, t½ ~7 days), PEGylation, and DAC can dramatically extend peptide half-lives. Use our Half-Life Visualizer to see concentration curves. See also: Pharmacokinetics.

HPLC (High-Performance Liquid Chromatography)

An analytical chemistry technique used to separate, identify, and quantify components in a mixture. In peptide analysis, HPLC is the standard method for measuring purity. The peptide sample is dissolved in a solvent and passed through a column; different components elute at different rates, producing a chromatogram. The main peak represents the target peptide, and purity is calculated as the area of the main peak divided by total peak area. A purity of >98% is considered pharmaceutical grade. See our COA guide for more.

Hypothalamus

A small region at the base of the brain that serves as the master regulator of the endocrine system. The hypothalamus produces several peptide hormones that control the pituitary gland, including GHRH (stimulates growth hormone release), somatostatin (inhibits growth hormone release), gonadotropin-releasing hormone (GnRH), and corticotropin-releasing hormone (CRH). Many peptide therapeutics work by mimicking or modulating hypothalamic peptide signals.

I

Incretin

A group of metabolic hormones that are released from the gut after eating and stimulate insulin secretion in a glucose-dependent manner. The two main incretins are GLP-1 and GIP. The "incretin effect" accounts for 50-70% of the insulin response to an oral glucose load, which is why oral glucose produces a stronger insulin response than the same amount of glucose given intravenously. Incretin-based therapies (GLP-1 receptor agonists and DPP-4 inhibitors) are a major class of type 2 diabetes and obesity treatments.

Intramuscular (IM)

A route of drug administration in which the medication is injected directly into skeletal muscle tissue. IM injections use longer needles (typically 1-1.5 inches) at a 90-degree angle. Common IM sites include the deltoid, vastus lateralis, and ventrogluteal muscles. IM injection provides faster absorption than subcutaneous injection due to the greater blood supply in muscle tissue. While most peptides are given subcutaneously, some formulations use the IM route. See our injection safety guide.

IU (International Unit)

A unit of measurement for the amount of a substance based on its biological activity or effect, rather than its mass. IUs are defined by the WHO and vary by substance. For example, 1 IU of growth hormone equals approximately 0.33 mg of somatotropin. For insulin, 1 IU equals approximately 0.0347 mg. When working with peptides, it is important to know whether a dose is specified in IU, micrograms (mcg), or milligrams (mg), as these are not directly interchangeable without knowing the conversion factor for the specific peptide.

L

Lyophilization

A freeze-drying process used to preserve peptides and proteins. The peptide solution is first frozen, then placed under high vacuum, causing the ice to sublimate (transition directly from solid to vapor without passing through a liquid phase). This produces a dry, porous cake or powder that retains the peptide's molecular structure and biological activity. Lyophilized peptides are much more stable than liquid formulations and can be stored for months or years before reconstitution. See our reconstitution guide.

M

Mass Spectrometry (MS)

An analytical technique that measures the mass-to-charge ratio of ions to identify and quantify molecules. In peptide analysis, mass spectrometry is used to confirm the identity of a peptide by comparing its observed molecular weight to the expected (theoretical) value calculated from the amino acid sequence. Common MS techniques for peptides include electrospray ionization (ESI-MS) and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF). See our COA guide for how to interpret MS results.

Melanocortin

A family of peptide hormones that includes alpha-MSH, beta-MSH, gamma-MSH, and ACTH, all derived from the precursor protein proopiomelanocortin (POMC). Melanocortins act through melanocortin receptors (MC1R through MC5R) and regulate diverse functions including skin pigmentation, appetite, sexual function, and immune modulation. PT-141 (bremelanotide) is a synthetic melanocortin receptor agonist that primarily targets MC4R, approved for hypoactive sexual desire disorder.

Molecular Weight

The sum of the atomic weights of all atoms in a molecule, typically expressed in daltons (Da) or grams per mole (g/mol). Peptide molecular weights range from approximately 200 Da (dipeptides) to approximately 5,000 Da (50-amino-acid peptides). Molecular weight is a key identity parameter on Certificates of Analysis and is measured by mass spectrometry. It also influences bioavailability, as larger peptides generally have lower oral absorption.

N

Neuropeptide

A peptide that functions as a neurotransmitter or neuromodulator in the nervous system. Neuropeptides are produced by neurons and released from nerve terminals to act on target cells. Examples include endorphins (natural pain-relieving peptides), substance P (involved in pain signaling), neuropeptide Y (involved in appetite regulation), and oxytocin (involved in social bonding and uterine contraction). Unlike classical neurotransmitters (such as dopamine and serotonin), neuropeptides are larger molecules that are synthesized in the cell body and transported to the nerve terminal.

O

Off-Label

The practice of prescribing an FDA-approved drug for a use, dose, population, or route of administration that is not included in the drug's approved labeling. Off-label prescribing is legal and common -- physicians are permitted to use their clinical judgment to prescribe approved drugs for unapproved indications. For example, low-dose naltrexone and certain compounded peptides are often used off-label. However, pharmaceutical manufacturers cannot market or promote drugs for off-label uses.

P

Peptide

A short chain of amino acids linked by peptide bonds, typically containing between 2 and approximately 50 amino acid residues. Peptides are distinguished from proteins by their shorter length, although the boundary is not absolute. Peptides are found naturally throughout the body as hormones, neurotransmitters, growth factors, and immune modulators. Synthetic peptides are used as therapeutics, research tools, and diagnostic agents. See our comprehensive introduction: Peptides 101.

Peptide Bond

A covalent chemical bond formed between the carboxyl group (-COOH) of one amino acid and the amino group (-NH2) of another through a condensation reaction (releasing one water molecule). Peptide bonds are the structural backbone of all peptides and proteins. They are planar and rigid, contributing to the overall three-dimensional structure of the molecule. Enzymes called peptidases (or proteases) break peptide bonds, which is why peptides are degraded in the digestive system and have short plasma half-lives.

Pharmacokinetics (PK)

The study of how the body processes a drug over time, often summarized as "what the body does to the drug." PK encompasses four processes: Absorption (how the drug enters the bloodstream), Distribution (how it spreads through body tissues), Metabolism (how it is broken down, primarily by enzymes), and Elimination (how it is removed from the body). Key PK parameters include bioavailability, half-life, volume of distribution, and clearance. Use our Half-Life Visualizer to explore peptide PK curves.

Pituitary Gland

A small, pea-sized gland located at the base of the brain, often called the "master gland" because it produces hormones that regulate many other endocrine glands. The anterior pituitary produces growth hormone, thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), and luteinizing hormone (LH), among others. The posterior pituitary releases oxytocin and vasopressin (ADH). Many peptide therapeutics work by stimulating or modulating pituitary hormone release via hypothalamic peptide pathways.

R

Receptor

A protein molecule on the surface of or inside a cell that binds to a specific substance (called a ligand) and initiates a cellular response. Receptors are highly specific -- they typically bind only one molecule or a closely related family of molecules. When a peptide agonist binds to its receptor, it triggers a signaling cascade inside the cell. Most peptide therapeutics work by binding to cell-surface receptors (G protein-coupled receptors, receptor tyrosine kinases, or cytokine receptors). Understanding receptor binding is fundamental to understanding peptide pharmacology.

Reconstitution

The process of dissolving a lyophilized (freeze-dried) peptide back into a liquid solution by adding a sterile solvent, typically bacteriostatic water. Reconstitution must be performed aseptically (clean technique) to prevent contamination. The water should be directed against the glass wall of the vial, not sprayed directly onto the powder, and the vial should be swirled gently -- never shaken. See our full reconstitution guide and reconstitution calculator.

Research Chemical

A chemical substance sold for laboratory research purposes, explicitly labeled "not for human consumption" or "for research use only." In the peptide space, many compounds that have not undergone FDA approval (such as BPC-157 and TB-500) are available as research chemicals. Purchasing and possessing research chemicals is generally legal, but their quality, purity, and sterility are not regulated to pharmaceutical standards. See our legal landscape guide.

S

Secretagogue

A substance that stimulates the secretion of another substance. In the peptide world, "secretagogue" most commonly refers to growth hormone secretagogues -- peptides or small molecules that stimulate the pituitary gland to release growth hormone. Growth hormone secretagogues include GH-releasing peptides (GHRPs like GHRP-2, GHRP-6, ipamorelin) and GHRH analogs (CJC-1295, sermorelin). The non-peptide MK-677 (ibutamoren) is also classified as a GH secretagogue.

Subcutaneous (SubQ)

A route of drug administration in which medication is injected into the layer of fat (adipose tissue) just beneath the skin and above the muscle. Subcutaneous injection is the most common administration route for peptide therapeutics, using thin needles (29G-31G) at a 45-90 degree angle. Common injection sites include the abdomen (2+ inches from the navel), anterior thigh, and upper arm. SubQ injection provides slower, more sustained absorption compared to intramuscular injection. See our injection safety guide and syringe guide.

T

Thymosin

A family of peptides originally isolated from the thymus gland. Thymosin alpha-1 (28 amino acids) is involved in immune system modulation and is FDA-approved in some countries for hepatitis B and as an immune adjuvant. Thymosin beta-4 (43 amino acids) promotes cell migration, wound healing, and tissue repair. TB-500 is a synthetic fragment (17 amino acids) of thymosin beta-4 that contains the active site responsible for its cell-migration and anti-inflammatory properties.

Titration

The practice of gradually adjusting the dose of a medication over time to find the optimal therapeutic dose while minimizing side effects. Titration is standard practice for many peptide therapies. For example, semaglutide is started at 0.25 mg weekly and titrated up every 4 weeks to the maintenance dose, allowing the body to acclimate and reducing gastrointestinal side effects. Our GLP-1 Titration Scheduler generates week-by-week titration plans. See also: Dose-Response.

W

WADA (World Anti-Doping Agency)

An international independent agency that coordinates the fight against doping in sport. WADA publishes an annually updated Prohibited List of substances and methods banned in competitive sport. Many peptides are on this list, including all growth hormone secretagogues, GH-releasing factors, BPC-157, and TB-500. Athletes subject to anti-doping testing must check the current Prohibited List before using any peptide. Therapeutic Use Exemptions (TUEs) may be available for medically necessary peptide treatments. See our legal landscape guide for more details.

Video Resources

These videos from trusted educators provide additional context on the topics covered in this guide.