Peptide glossary.

A molecule that binds a receptor and activates it, producing a biological response — the opposite of an antagonist, which blocks the receptor.

The building block of peptides and proteins. Peptides are short chains of amino acids linked by peptide bonds; sequence determines structure and activity.

A molecule that binds a receptor without activating it, blocking the natural ligand from producing its effect.

Sterile water with 0.9% benzyl alcohol that suppresses bacterial growth, allowing a reconstituted multi-use vial to be stored and drawn from over time.

The fraction of an administered dose that reaches systemic circulation intact. Subcutaneous peptides often have higher bioavailability than oral forms, which are degraded in the gut.

A specific situation or condition in which a drug or intervention should not be used because the risk outweighs any benefit.

A cell's reduction in receptor number or sensitivity after sustained stimulation — a reason many peptide protocols cycle on and off.

ATLAS labels each claim A–D or P: A = RCT/meta-analysis, B = large cohort/consistent trials, C = small trial/mechanistic, D = expert/textbook/regulatory, P = preclinical/animal.

A compound that triggers growth hormone release, including ghrelin-mimetic peptides (ipamorelin, GHRP-2/6, hexarelin) acting on the GHS-R receptor.

A peptide that mimics growth-hormone-releasing hormone (e.g., sermorelin, tesamorelin, CJC-1295), stimulating pituitary GH release while preserving feedback control.

A drug that activates the glucagon-like peptide-1 receptor to enhance insulin secretion, slow gastric emptying, and reduce appetite (e.g., semaglutide).

The time for the plasma concentration of a compound to fall by half. It drives dosing frequency — short half-life peptides are dosed more often.

The tendency of a peptide or protein to provoke an immune response, including anti-drug antibodies that can reduce efficacy or cause reactions.

A gut hormone (GLP-1, GIP) released after eating that amplifies insulin secretion. Incretin-based peptides are central to modern metabolic therapy.

A unit of measure based on biological activity rather than mass, used for some hormones. Not interchangeable with mg/mcg without a conversion factor.

The breakdown of stored fat into free fatty acids and glycerol — a target mechanism for several metabolic peptides.

Freeze-dried into a stable powder for storage and shipping; reconstituted with liquid before use.

One millionth of a gram; 1000 mcg = 1 mg. Most peptide doses are expressed in mcg.

The mass of one molecule in daltons. Higher molecular weight peptides generally cannot be absorbed orally intact.

A peptide used by neurons to signal, modulating mood, cognition, appetite, or stress responses (e.g., selank, semax, oxytocin).

Use of an approved drug for an indication, dose, or population not specified on its approved label — legal for clinicians but not promoted by manufacturers.

The covalent amide link between the carboxyl group of one amino acid and the amino group of the next, forming the backbone of a peptide chain.

What a drug does to the body — its mechanism, receptor effects, and dose-response relationship.

What the body does to a drug — absorption, distribution, metabolism, and excretion over time.

A protein that binds a signaling molecule and triggers a cellular response. Peptides act largely by binding specific cell-surface receptors.

Dissolving a lyophilized (freeze-dried) peptide powder in a liquid (usually bacteriostatic water) to create an injectable solution at a known concentration.

A labeling category meaning a product is sold for laboratory research and is not approved or intended for human or clinical use.

Any substance that causes another substance — typically a hormone — to be secreted.

Injection into the fat layer just under the skin — the most common route for self-administered peptides, using a short fine needle.

Gradually adjusting a dose up or down to reach the lowest effective amount while limiting side effects — standard for incretin peptides.

An insulin syringe marked so 100 units equals 1 mL; 1 unit = 0.01 mL. Peptide draw volumes are usually read in these units.

World Anti-Doping Agency categories. S0 covers non-approved substances; S2 covers peptide hormones and growth factors — both prohibited in sport.