● GLOSSARY91 TERMS · DEFINED

The glossary.

91 definitions spanning pharmacology, research methods, regulatory terms, and practical peptide science.

Key terms used in peptide science and research — the jargon you’ll run into across profiles, forum threads, and the primary literature. Jump to a letter, or scroll.

§ A

8 terms

Amino Acid: Organic molecules that serve as the building blocks of peptides and proteins. There are 20 standard amino acids encoded by DNA.
Amphipathic: Having both hydrophilic and hydrophobic regions. Many antimicrobial peptides like LL-37 have amphipathic structures that insert into cell membranes.
Angiogenesis: Formation of new blood vessels from existing ones. Peptides like BPC-157 and TB-500 promote angiogenesis for wound healing and tissue repair.
Agonist: A molecule that binds to a receptor and activates it. Semaglutide is a GLP-1 receptor agonist; ipamorelin is a ghrelin receptor agonist.
Antagonist: A molecule that binds to a receptor and blocks its activation. Used in research to study receptor-specific peptide effects.
Acetylation: Addition of an acetyl group to a peptide's N-terminus. Increases stability and can alter receptor binding. N-Acetyl Semax Amidate is an acetylated variant.
Allosteric Modulator: A molecule that binds a receptor at a site other than the active site, changing its shape and activity. Can be positive (enhancing) or negative (reducing).
Amidation: Modification of a peptide's C-terminus with an amide group. Increases stability and receptor binding affinity. Common in synthetic peptide manufacturing.

§ B

5 terms

Bioavailability: The proportion of a substance that enters systemic circulation. Subcutaneous injection typically offers higher bioavailability than oral administration for peptides.
Bioregulator: Short peptides (2-4 amino acids) that regulate gene expression at the epigenetic level. Term coined by Vladimir Khavinson. Examples: Epithalon, Thymalin.
BDNF: Brain-Derived Neurotrophic Factor. Supports neuronal survival, growth, and plasticity. Increased by peptides like Semax, Selank, and Dihexa.
Bacteriostatic Water: Sterile water containing 0.9% benzyl alcohol to inhibit bacterial growth. The standard solvent for reconstituting lyophilized peptides.
BPC: Body Protection Compound. A pentadecapeptide (15 amino acids) isolated from human gastric juice. BPC-157 is the most studied fragment.

§ C

12 terms

CAS Number: Chemical Abstracts Service registry number — a unique identifier for chemical substances, used to unambiguously identify peptide compounds.
Cathelicidin: A family of antimicrobial peptides. LL-37 is the only human cathelicidin, part of the innate immune system and regulated by vitamin D.
Compounding Pharmacy: A pharmacy that creates customized medications. 503A pharmacies serve individuals; 503B facilities produce larger batches under FDA oversight.
Cycling: Using a peptide for a defined period followed by a break (e.g., 8 weeks on, 4 off). Prevents receptor desensitization and maintains effectiveness.
Circadian Rhythm: The body's 24-hour internal clock. Peptides like DSIP and delta-sleep-inducing peptides interact with circadian regulation.
Clearance: The rate at which a substance is removed from the body. Renal clearance is the primary elimination route for most peptides.
C-Terminal: The end of a peptide chain with a free carboxyl group (-COOH). Peptide sequences are written from N-terminal (left) to C-terminal (right).
Category 1 (FDA): FDA classification for bulk drug substances that may be used in compounding. Category 1 peptides can be legally compounded by 503A/503B pharmacies.
Category 2 (FDA): FDA classification for substances that may NOT be compounded due to safety concerns. As of 2024, includes BPC-157, TB-500, and 17 other peptides.
Certificate of Analysis (COA): Document from a laboratory confirming a peptide's identity, purity, and absence of contaminants. Third-party COAs from independent labs are the gold standard.
cGMP: Current Good Manufacturing Practice. FDA regulations ensuring pharmaceutical products are consistently produced to quality standards. Required for 503B facilities.
Cyclization: Forming a ring structure within a peptide by linking side chains or termini. Increases metabolic stability and receptor selectivity. Used in PT-141 (bremelanotide).

§ D

6 terms

Dalton (Da): Unit of molecular mass. Peptides range from ~300 Da (tripeptides) to ~5,000 Da. Proteins are generally over 10,000 Da.
Desensitization: Reduced receptor responsiveness after prolonged agonist exposure. Hexarelin causes GHS-R1a desensitization, requiring cycling.
Disulfide Bond: A covalent bond between two cysteine residues. Stabilizes peptide 3D structure. Oxytocin contains one disulfide bond; insulin has three.
Dose-Response Curve: Graph showing the relationship between drug dose and biological effect. Helps determine effective dose, ceiling dose, and therapeutic window for peptides.
Double-Blind Study: Clinical trial where neither participants nor researchers know who receives the active treatment vs placebo. The gold standard for reducing bias.
DSHEA: Dietary Supplement Health and Education Act (1994). Defines supplements vs drugs. Most injectable peptides do NOT qualify as supplements under DSHEA.

§ E

2 terms

Endogenous: Produced naturally within the body. GLP-1, oxytocin, and endorphins are endogenous peptides. Contrast with exogenous (externally administered).
Exogenous: Introduced from outside the body. Synthetic peptides like semaglutide are exogenous analogs of natural peptides.

§ F

3 terms

FAK-Paxillin Pathway: Focal Adhesion Kinase signaling pathway governing cell adhesion and tissue remodeling. Activated by BPC-157 to promote tissue repair.
FDA 503A/503B: FDA classifications for compounding pharmacies. 503A serves individual prescriptions; 503B facilities produce larger batches under stricter oversight.
First-Pass Metabolism: Breakdown of an orally administered substance by the liver before reaching systemic circulation. Most peptides have poor oral bioavailability due to first-pass metabolism.

§ G

6 terms

GH (Growth Hormone): A 191-amino-acid peptide hormone from the anterior pituitary. Many peptides (Ipamorelin, CJC-1295, Hexarelin) stimulate natural GH release.
GHRH: Growth Hormone-Releasing Hormone. A hypothalamic peptide stimulating GH synthesis and release. CJC-1295 and Sermorelin are synthetic GHRH analogs.
GHRP: Growth Hormone Releasing Peptide. Synthetic peptides activating the ghrelin receptor to release GH. Examples: Ipamorelin, Hexarelin.
GLP-1: Glucagon-Like Peptide-1. An incretin hormone regulating appetite and blood sugar. Semaglutide, liraglutide, and tirzepatide are GLP-1 agonists.
Ghrelin: The 'hunger hormone' produced primarily in the stomach. Activates GHS-R1a receptors. Ipamorelin and hexarelin mimic ghrelin's GH-releasing action.
Gauge (Needle): Measure of needle diameter — higher gauge = thinner needle. Subcutaneous peptide injections typically use 27-31 gauge insulin syringes.

§ H

2 terms

Half-life: Time for a substance's concentration to decrease by half. Determines dosing frequency — from minutes (Semax) to days (semaglutide).
HPG Axis: Hypothalamic-Pituitary-Gonadal Axis. The hormonal feedback loop controlling reproductive hormones. Kisspeptin is its master upstream regulator.

§ I

7 terms

IGF-1: Insulin-like Growth Factor 1. Produced by the liver in response to GH. Mediates many anabolic effects including muscle growth.
Incretin: Gut hormones that stimulate insulin secretion after eating. GLP-1 and GIP are the two main incretins targeted by weight-loss peptides.
Intramuscular (IM): Injection into muscle tissue. Some peptides (TB-500, BPC-157) can be administered IM for localized effect near injury sites.
In Vitro: Experiments conducted in a controlled environment outside a living organism (e.g., cell cultures, test tubes). Latin for 'in glass.'
In Vivo: Experiments conducted in living organisms (typically animal models). Provides more complete pharmacological data than in vitro but precedes human trials.
Injection Site Rotation: Alternating injection locations (abdomen, thigh, upper arm) to prevent lipodystrophy, scarring, and inconsistent absorption at overused sites.
Investigational New Drug (IND): FDA application required before testing an unapproved drug in humans. Most research peptides have not filed INDs, limiting legal clinical use.

§ L

2 terms

Lyophilization: Freeze-drying process preserving peptides as powder. Must be reconstituted with bacteriostatic water before injection.
Loading Phase: An initial period of higher or more frequent dosing to build tissue levels before transitioning to a maintenance dose. Common with TB-500.

§ M

3 terms

Melanocortin Receptor: G-protein coupled receptors (MC1R-MC5R) binding melanocortins. MC1R controls pigmentation, MC4R affects appetite and sexual arousal.
Mitochondrial-Derived Peptide: Peptides encoded by mitochondrial DNA. MOTS-c and Humanin are the most studied, linking mitochondrial function to metabolism.
Meta-Analysis: Statistical method combining results from multiple studies to identify patterns. Provides stronger evidence than individual trials. Common in GLP-1 efficacy research.

§ N

4 terms

NF-kB: Nuclear Factor kappa B. A master transcription factor controlling inflammation. Inhibited by anti-inflammatory peptides like KPV.
Neuropeptide: Peptides acting as neurotransmitters or neuromodulators in the nervous system. Examples: substance P, neuropeptide Y, selank, semax.
N-Terminal: The end of a peptide chain with a free amino group (-NH2). The starting point of peptide synthesis and sequence notation.
New Drug Application (NDA): FDA application for marketing approval of a new drug. Requires extensive clinical trial data. Semaglutide and tirzepatide have approved NDAs.

§ P

11 terms

Peptide: A short chain of amino acids (typically 2-50) linked by peptide bonds. Smaller than proteins. Function as hormones, neurotransmitters, and growth factors.
Peptide Bond: The covalent bond between two amino acids, formed when one's carboxyl group reacts with another's amino group, releasing water.
PEGylation: Attaching polyethylene glycol chains to a peptide to extend its half-life and reduce immunogenicity. Used in CJC-1295 DAC.
Preclinical: Research stage before human trials, including in vitro (cell) and in vivo (animal) studies. Most research peptides have only preclinical data.
p-value: Statistical measure of the probability that observed results occurred by chance. p < 0.05 is conventionally considered statistically significant in peptide research.
Peptide Purity: Percentage of the target peptide in a sample, measured by HPLC. Research-grade is typically >95%; pharmaceutical-grade is >99%.
Pharmacodynamics: The study of what a drug does to the body — mechanism of action, receptor binding, and biological effects. Complements pharmacokinetics.
Pharmacokinetics: The study of what the body does to a drug — absorption, distribution, metabolism, and excretion (ADME). Determines dosing frequency and route.
Phase I/II/III Trial: Stages of human clinical testing. Phase I tests safety in small groups; Phase II tests efficacy; Phase III confirms benefit in large populations before FDA approval.
Placebo-Controlled: Study design where a control group receives an inactive substance. Essential for determining whether a peptide's effects exceed the placebo response.
Prodrug: An inactive compound that is metabolized into an active drug inside the body. Some peptide formulations use prodrug strategies to improve oral bioavailability.

§ R

4 terms

Reconstitution: Dissolving lyophilized peptide powder in bacteriostatic water. Gentle swirling (not shaking) preserves peptide integrity.
Receptor Selectivity: A peptide's ability to bind one receptor type without affecting others. Ipamorelin is highly selective for GHS-R1a over cortisol receptors.
Receptor Agonist: A molecule that binds and activates a receptor, mimicking the natural ligand. Semaglutide is a GLP-1 receptor agonist with 94% homology to native GLP-1.
Receptor Antagonist: A molecule that binds a receptor without activating it, blocking the natural ligand. CGRP antagonists (gepants) treat migraines by blocking CGRP signaling.

§ S

5 terms

Secretagogue: A substance promoting secretion. Growth hormone secretagogues stimulate the pituitary to release GH. Examples: Ipamorelin, Hexarelin, CJC-1295.
Subcutaneous Injection: Injection into fatty tissue between skin and muscle. The most common peptide administration route, using short insulin-type needles.
SPPS: Solid-Phase Peptide Synthesis. The standard method for manufacturing synthetic peptides, pioneered by Bruce Merrifield in 1963.
Statistical Significance: A result unlikely to have occurred by chance alone (typically p < 0.05). Does not necessarily mean clinical significance — effect size also matters.
Systematic Review: A comprehensive, methodical review of all available evidence on a specific research question. Often includes a meta-analysis of combined data.

§ T

7 terms

Telomerase: Enzyme adding telomeric repeat sequences to chromosome ends, counteracting age-related shortening. Activated by Epithalon.
Telomere: Protective DNA caps at chromosome ends. Shorten with each cell division. Length is considered a biomarker of biological aging.
Titration: Gradually adjusting dosage to find the optimal dose while minimizing side effects. Most peptide protocols start low and titrate upward.
Thymosin: A family of small proteins first isolated from the thymus gland. Thymosin alpha-1 modulates immunity; thymosin beta-4 (TB-500) promotes healing.
Tachyphylaxis: Rapid decrease in drug response after repeated doses. Distinct from tolerance (which develops gradually). Can occur with some GH secretagogues.
Therapeutic Index: Ratio between a drug's toxic dose and therapeutic dose. A wide therapeutic index means a larger margin of safety. Most peptides have favorable therapeutic indices.
Third-Party Testing: Independent laboratory analysis of a peptide's purity, identity, and contaminant levels. Critical for research peptides purchased from unregulated vendors.

§ U

1 term

USP: United States Pharmacopeia. Sets quality standards for drugs, including peptides. USP-grade bacteriostatic water and syringes meet these standards.

§ V

1 term

Vial: A sealed glass container holding lyophilized peptide powder. Typical sizes are 2mg, 5mg, and 10mg. The rubber stopper allows syringe access without opening.

§ W

1 term

WADA: World Anti-Doping Agency. Maintains the prohibited list for competitive sport. Many peptides including growth hormone secretagogues, IGF-1, and TB-500 are banned under S2 (peptide hormones and growth factors).

§ X

1 term

Xenopeptide: A peptide derived from a non-human organism. Examples include magainin (frog skin) and cecropin (insect hemolymph). Studied for antimicrobial properties.

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