Peptide Glossary
Essential terminology for understanding peptides, from amino acids to vasoactive compounds.
A
Amino Acid
Organic molecules that serve as the building blocks of peptides and proteins. There are 20 standard amino acids, each with a unique side chain that determines its properties. Peptides are chains of amino acids linked by peptide bonds.
Anabolism
The metabolic process of building complex molecules from simpler ones, requiring energy. In the context of peptides, anabolic peptides promote tissue growth, muscle building, and repair. The opposite of catabolism.
B
Bacteriostatic Water
Sterile water containing 0.9% benzyl alcohol as a preservative to inhibit bacterial growth. Used to reconstitute lyophilized peptides. Unlike plain sterile water, it allows for multiple withdrawals from a vial over weeks.
Bioavailability
The proportion of a substance that enters systemic circulation and is available for biological activity. Injectable peptides typically have near 100% bioavailability, while oral peptides may have significantly lower bioavailability due to digestive breakdown.
BPC (Body Protection Compound)
A peptide sequence derived from human gastric juice. BPC-157, the most studied variant, is a 15-amino-acid peptide known for remarkable healing properties on tendons, ligaments, muscles, gut lining, and the nervous system.
C
Catabolism
The metabolic process of breaking down complex molecules into simpler ones, releasing energy. Catabolic states (from stress, fasting, or overtraining) can lead to muscle breakdown. Many peptides aim to shift the body toward anabolism.
Cycling
The practice of using a peptide for a set period ('on' cycle) followed by a break ('off' cycle). For example, '8 weeks on, 4 weeks off.' Cycling prevents receptor desensitization and maintains peptide effectiveness over time.
D
Desensitization
A reduction in receptor responsiveness after prolonged or repeated exposure to a stimulus. With peptides, continuous use without breaks can cause receptors to down-regulate, reducing the peptide's effectiveness. This is why cycling is recommended.
Down-regulation
A cellular process where the number or sensitivity of receptors decreases in response to chronic stimulation. When a peptide continuously activates a receptor, the cell may reduce receptor expression to maintain homeostasis.
Drug Affinity Complex (DAC)
A chemical modification that extends a peptide's half-life by promoting binding to albumin in the blood. CJC-1295 with DAC has a half-life of ~8 days versus ~30 minutes without DAC, allowing less frequent dosing.
E
Endogenous
Produced naturally within the body. Many research peptides are synthetic versions of endogenous peptides. For example, ghrelin is an endogenous peptide, and GHRPs are synthetic analogs designed to mimic its GH-releasing effects.
Exogenous
Originating from outside the body. All administered peptides are exogenous substances. Exogenous peptides can supplement, mimic, or enhance the effects of endogenous peptides. The opposite of endogenous.
G
GH (Growth Hormone)
A 191-amino-acid protein hormone produced by the pituitary gland. GH regulates growth, body composition, metabolism, and cell repair. Many peptides (GHRPs, GHRHs) work by stimulating natural GH release rather than replacing it directly.
GHRH (Growth Hormone Releasing Hormone)
A class of peptides that stimulate the pituitary gland to release growth hormone by mimicking the body's natural GHRH signal. Examples include CJC-1295 and Sermorelin. Often stacked with GHRPs for synergistic GH release.
GHRP (Growth Hormone Releasing Peptide)
A class of synthetic peptides that stimulate GH release through the ghrelin receptor (GHS-R). Unlike GHRHs, GHRPs create a GH 'pulse' and also suppress somatostatin. Examples include Ipamorelin, GHRP-2, and GHRP-6.
GLP-1 (Glucagon-Like Peptide-1)
An incretin hormone produced in the gut that stimulates insulin secretion, suppresses glucagon, slows gastric emptying, and reduces appetite. GLP-1 receptor agonists (semaglutide, tirzepatide) are among the most successful peptide drugs for diabetes and obesity.
H
Half-life
The time it takes for the concentration of a substance in the body to decrease by half. Half-life determines dosing frequency: short half-life peptides (minutes) may need multiple daily doses, while modified peptides (e.g., with DAC) can last days.
HGH (Human Growth Hormone)
Synthetic recombinant human growth hormone, identical to the 191-amino-acid hormone produced by the pituitary. Prescription HGH (Humatrope, Genotropin) is FDA-approved for GH deficiency. Many people use GH-releasing peptides as a safer, more natural alternative.
Homeostasis
The body's tendency to maintain stable internal conditions (temperature, pH, hormone levels, etc.). When exogenous peptides alter a system, the body may compensate through feedback mechanisms — this is why cycling and monitoring are important.
I
IGF-1 (Insulin-like Growth Factor 1)
A hormone structurally similar to insulin, primarily produced by the liver in response to GH. IGF-1 mediates many of GH's anabolic effects including muscle growth, bone density, and tissue repair. GH-releasing peptides indirectly raise IGF-1 levels.
Injection (Subcutaneous / Intramuscular)
Subcutaneous (subQ) injections go into the fat layer under the skin — most common for peptides, using insulin syringes. Intramuscular (IM) injections go into muscle tissue — less common for peptides, used for some like Cerebrolysin. SubQ is preferred for convenience and slower absorption.
L
Lyophilized
Freeze-dried. Most research peptides are sold in lyophilized powder form for stability. The freeze-drying process removes water while preserving the peptide's structure. Must be reconstituted with bacteriostatic water before use.
M
Melanocortin
A family of peptide hormones that bind to melanocortin receptors (MC1R–MC5R), regulating skin pigmentation, appetite, sexual function, and inflammation. Melanotan I, Melanotan II, and PT-141 are synthetic melanocortin analogs.
N
O
Off-label
Using a medication for a purpose other than its FDA-approved indication. For example, using semaglutide (approved for diabetes) for weight loss was initially off-label before Wegovy's approval. Many peptide uses are considered off-label or unapproved.
P
Peptide Bond
The covalent chemical bond formed between the carboxyl group of one amino acid and the amino group of another, with the loss of a water molecule. Peptide bonds link amino acids together to form peptide chains. They are strong and stable under physiological conditions.
R
Reconstitution
The process of dissolving lyophilized (freeze-dried) peptide powder with a solvent, typically bacteriostatic water. Proper reconstitution involves slowly adding water to the vial wall (not directly onto powder), then gently swirling until dissolved.
S
Secretagogue
A substance that stimulates the secretion of another substance. Growth hormone secretagogues (GHS) stimulate the pituitary to release GH naturally, as opposed to injecting synthetic GH directly. Examples include GHRPs and GHRHs.
Senolytic
A class of compounds that selectively clear senescent ('zombie') cells — old, damaged cells that stop dividing but don't die, releasing inflammatory signals. FOXO4-DRI is a senolytic peptide that disrupts the survival mechanism of senescent cells.
Subcutaneous
Relating to the layer of tissue just beneath the skin. Subcutaneous injection is the most common administration route for peptides, delivering the peptide into fatty tissue for gradual absorption into the bloodstream.
T
Telomerase
An enzyme that adds DNA sequence repeats to the ends of chromosomes (telomeres), counteracting the shortening that occurs with each cell division. Epitalon is a peptide that stimulates telomerase activity, potentially slowing cellular aging.
Thymosin
A family of peptides originally isolated from the thymus gland, involved in immune regulation and tissue repair. Key members include Thymosin Alpha-1 (immune modulator), Thymosin Beta-4/TB-500 (tissue repair), and Thymalin (thymic function restoration).
V
Vasoactive
Having an effect on blood vessel diameter. Vasoactive peptides can dilate (vasodilators) or constrict (vasoconstrictors) blood vessels, affecting blood pressure and tissue perfusion. VIP (Vasoactive Intestinal Peptide) is a key example regulating blood flow and gut function.