Peptide Terminology Glossary
Peptide research comes with a steep learning curve of scientific terminology. Whether you are reading a study abstract, reviewing a Certificate of Analysis, or trying to understand a dosing protocol, unfamiliar terms can create confusion. This glossary defines over 80 of the most common terms in peptide science, pharmacology, and practical use — in plain, accessible language.
Last updated: 2026-03-03
Steps
1
Use the alphabetical index to find your term
The glossary is organized alphabetically. Use the section headings (A-C, D-G, etc.) or your browser's find function (Ctrl+F / Cmd+F) to jump to a specific term.
2
Read the definition and follow cross-references
Many terms reference related concepts. Follow the internal links to build a complete understanding of interconnected ideas.
3
Bookmark this page for future reference
Keep this glossary handy while reading research papers, product labels, or guides on this site.
A – C
- Agonist
- A molecule that binds to a receptor and activates it, triggering a biological response. For example, semaglutide is a GLP-1 receptor agonist — it binds the GLP-1 receptor and activates insulin release and appetite suppression.
- Amino Acid
- The building blocks of peptides and proteins. There are 20 standard amino acids. Peptides are chains of 2-50 amino acids; anything longer is typically classified as a protein.
- Anabolic
- Relating to the building of complex molecules from simpler ones — the opposite of catabolic. Growth hormone secretagogues like ipamorelin promote anabolic processes including muscle protein synthesis.
- Antagonist
- A molecule that binds to a receptor but does not activate it, blocking the receptor from being activated by its natural ligand or an agonist.
- Bacteriostatic Water (BAC Water)
- Sterile water containing 0.9% benzyl alcohol as a preservative. The standard solvent for reconstituting peptides. The benzyl alcohol inhibits bacterial growth, allowing multi-dose use from a single vial.
- Bioavailability
- The fraction of an administered dose that reaches systemic circulation in its active form. Subcutaneous injection provides high bioavailability; oral peptides have much lower bioavailability due to gastrointestinal degradation.
- Bioidentical
- A synthetic compound that is chemically identical to a naturally occurring molecule in the body. BPC-157 is a synthetic version of a peptide fragment naturally found in human gastric juice.
- BPC (Body Protection Compound)
- A peptide originally isolated from human gastric juice. BPC-157 is a 15-amino-acid fragment studied for wound healing, gut protection, and anti-inflammatory effects.
- Certificate of Analysis (COA)
- A document from an analytical laboratory confirming the identity, purity, and quality of a peptide batch. See our guide to choosing a peptide source for how to read and verify COAs.
- CJC-1295
- A synthetic analog of growth hormone-releasing hormone (GHRH). Available in two forms: with DAC (Drug Affinity Complex) for extended half-life, and without DAC (also called Modified GRF 1-29) for shorter, pulsatile release. See our CJC-1295 compound page.
- Compounding Pharmacy
- A pharmacy that creates custom medications (including peptides) tailored to individual patient needs. Regulated under FDA sections 503A (patient-specific) or 503B (outsourcing facilities). The 2024 Category 2 decision restricted several peptides from compounding.
- Cycling
- The practice of using a peptide for a set period ("on" phase) followed by a break ("off" phase). Cycling helps maintain receptor sensitivity and reduce cumulative side effects. See our peptide cycling guide.
D – G
- DAC (Drug Affinity Complex)
- A chemical modification added to CJC-1295 that binds to albumin in the blood, extending the peptide's half-life from minutes to days. CJC-1295 with DAC provides continuous GH elevation; without DAC, it creates pulsatile GH release.
- Degradation
- The breakdown of a peptide's molecular structure, reducing or eliminating its biological activity. Heat, light, pH changes, and bacterial contamination are common causes. Proper storage minimizes degradation.
- Desensitization (Receptor Downregulation)
- A reduction in receptor number or sensitivity in response to continuous stimulation. This is the primary reason peptides like GH secretagogues require cycling.
- Diluent
- The liquid used to dissolve a lyophilized peptide. Bacteriostatic water is the most common diluent for research peptides.
- Dosage
- The amount of peptide administered per dose, typically measured in micrograms (mcg) or milligrams (mg). See our peptide dosage chart for compound-specific ranges.
- Endotoxin
- Toxic components from the outer membrane of gram-negative bacteria. Endotoxin contamination in injectable peptides can cause fever, inflammation, and serious adverse reactions. LAL (Limulus amebocyte lysate) testing detects endotoxins.
- Exogenous
- Originating from outside the body. Injected peptides are exogenous, as opposed to endogenous peptides your body produces naturally.
- GH (Growth Hormone)
- A peptide hormone produced by the pituitary gland. It promotes growth, cell reproduction, and regeneration. GH secretagogues stimulate the body to produce more of its own GH.
- GHRH (Growth Hormone-Releasing Hormone)
- A hypothalamic hormone that stimulates the pituitary to release GH. Sermorelin and CJC-1295 are synthetic GHRH analogs.
- GHRP (Growth Hormone-Releasing Peptide)
- A class of synthetic peptides that stimulate GH release through the ghrelin receptor (GHSR). Examples include GHRP-2, GHRP-6, hexarelin, and ipamorelin.
- GLP-1 (Glucagon-Like Peptide-1)
- An incretin hormone that stimulates insulin secretion, suppresses glucagon, slows gastric emptying, and reduces appetite. GLP-1 receptor agonists (semaglutide, tirzepatide) are FDA-approved for diabetes and weight management. See our GLP-1 guide.
H – L
- Half-Life
- The time it takes for the concentration of a peptide in the body to decrease by half. Half-life determines dosing frequency — shorter half-life peptides need more frequent dosing. Ipamorelin has a half-life of ~2 hours; CJC-1295 with DAC has a half-life of ~6-8 days.
- HPLC (High-Performance Liquid Chromatography)
- An analytical technique used to separate, identify, and quantify components in a mixture. HPLC purity is the primary quality metric on a peptide COA. Research-grade peptides should show ≥98% HPLC purity.
- Hydrophilic
- Water-loving; dissolves easily in water. Most peptides are hydrophilic and dissolve readily in bacteriostatic water.
- Hydrophobic
- Water-repelling; does not dissolve easily in water. Some peptides require acetic acid or other solvents for reconstitution due to hydrophobic amino acid sequences.
- IGF-1 (Insulin-Like Growth Factor 1)
- A hormone produced primarily by the liver in response to growth hormone. IGF-1 mediates many of GH's anabolic effects. Blood IGF-1 levels are used to assess GH status and monitor GH secretagogue therapy. See our IGF-1 LR3 compound page.
- Intramuscular (IM) Injection
- Injection into the muscle tissue. Less common for peptides than subcutaneous injection but used for some compounds. Deeper needle insertion required (typically 1-1.5 inches). See our injection guide.
- In Vitro
- Experiments conducted outside a living organism — in test tubes, petri dishes, or cell cultures. "In vitro" results do not necessarily translate to in vivo (living organism) effects.
- In Vivo
- Experiments conducted in a living organism (animal studies or human clinical trials). In vivo evidence is stronger than in vitro because it accounts for absorption, metabolism, and systemic interactions.
- Lyophilization (Freeze-Drying)
- The process of removing water from a peptide solution by freezing it and then reducing pressure to allow the ice to sublimate directly into vapor. This creates a stable powder that can be stored long-term and reconstituted when needed.
- Ligand
- A molecule that binds to a specific receptor. Peptides are ligands for their target receptors. An agonist ligand activates the receptor; an antagonist ligand blocks it.
M – P
- Mass Spectrometry (MS)
- An analytical technique that measures the mass-to-charge ratio of molecules. Used in COAs to confirm that a peptide's molecular weight matches the expected value, verifying its identity.
- mcg (Microgram)
- One millionth of a gram (0.001 mg). Most peptide doses are measured in micrograms. 1 mg = 1,000 mcg.
- Metabolism
- The body's process of breaking down substances. Peptides are metabolized by enzymes (proteases and peptidases), which is why oral bioavailability is typically low and injection is preferred.
- mg (Milligram)
- One thousandth of a gram. Peptide vials are typically sold in milligram quantities (e.g., 5mg, 10mg). See our peptide calculator for concentration math.
- Neuropeptide
- A peptide that acts as a neurotransmitter or neuromodulator in the nervous system. Semax, Selank, and DSIP are examples of neuropeptides used in research.
- Off-Label Use
- Using an FDA-approved medication for a purpose, population, or dosage not specified in its approved labeling. Physicians can legally prescribe off-label; manufacturers cannot promote off-label use.
- Peptide
- A chain of 2-50 amino acids linked by peptide bonds. Shorter chains are dipeptides (2), tripeptides (3), or oligopeptides (few). Longer chains (50+) are generally called proteins. See our What Are Peptides? guide for a full introduction.
- Peptide Bond
- The chemical bond that links amino acids together in a peptide chain. It forms between the carboxyl group of one amino acid and the amino group of the next, with the release of a water molecule.
- Pharmacokinetics (PK)
- The study of how the body absorbs, distributes, metabolizes, and excretes a drug. PK data determines dosing frequency, administration route, and onset/duration of action.
- Pituitary Gland
- A pea-sized gland at the base of the brain that produces and releases several hormones, including growth hormone. GH secretagogues work by stimulating the pituitary to release more GH.
- Protease
- An enzyme that breaks down proteins and peptides by cleaving peptide bonds. Proteases in the digestive system are why most peptides cannot be taken orally.
- Protocol
- A structured plan for peptide use including compound selection, dosage, frequency, cycle length, and timing. Our dosage guide provides protocol frameworks for common compounds.
R – Z
- Receptor
- A protein on the cell surface or inside the cell that binds specific molecules (ligands) and triggers a biological response. Each peptide has a target receptor — e.g., ipamorelin targets GHSR (growth hormone secretagogue receptor).
- Reconstitution
- The process of dissolving lyophilized (freeze-dried) peptide powder in a solvent (usually bacteriostatic water) to create an injectable solution. See our complete reconstitution guide.
- Research Peptide
- A peptide sold for laboratory/research purposes that has not been FDA-approved for human therapeutic use. Most peptides discussed on this site fall into this category. They are labeled "not for human consumption."
- Secretagogue
- A substance that stimulates the secretion of another substance. GH secretagogues (sermorelin, ipamorelin, CJC-1295) stimulate the pituitary gland to secrete growth hormone.
- Serum
- The liquid component of blood after clotting factors are removed. "Serum levels" of a peptide or hormone refer to the concentration measured in a blood test.
- Stacking
- Using two or more peptides simultaneously for complementary or synergistic effects. Common stacks include CJC-1295 + ipamorelin (GH release) and BPC-157 + TB-500 (healing). See our stacking guide.
- Subcutaneous (SubQ) Injection
- Injection into the fatty tissue layer just beneath the skin. The most common administration route for research peptides. Typically uses 29-31 gauge, 1/2 inch insulin syringes. See our injection guide.
- Synthesis
- The manufacturing process of creating peptides. Solid-phase peptide synthesis (SPPS) is the standard method, where amino acids are added one at a time to a growing chain anchored to a solid support.
- Tachyphylaxis
- A rapid decrease in response to a drug after repeated doses — essentially acute desensitization. Hexarelin is known for relatively fast tachyphylaxis, which is why shorter cycles are recommended.
- Upregulation
- An increase in the number of receptors on a cell surface, typically in response to reduced ligand stimulation. The off phase of peptide cycling promotes receptor upregulation (resensitization).
- Vial
- A small glass container sealed with a rubber stopper and aluminum crimp cap, used to store lyophilized or reconstituted peptides. Standard peptide vials are 2-3 mL in volume.
- WADA (World Anti-Doping Agency)
- The international organization that sets the list of substances prohibited in competitive sports. Most GH secretagogues, peptide hormones, and growth factors are on the WADA prohibited list. See our legality guide for details.
Explore Further
- What are peptides? — beginner-friendly introduction
- Beginner's guide to peptides — getting started checklist
- How to reconstitute peptides — step-by-step mixing guide
- How to inject peptides — injection technique and site rotation
- Peptide dosage chart — dosing ranges for every compound
- Peptide calculator — reconstitution and dosing math