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Peptide glossary

82 peptide, biochemistry, pharmacology, and sourcing terms in plain English. From GLP-1 agonist and isoelectric point to certificate of analysis and bacteriostatic water.

Biochemistry

Amino acid: The building block of peptides and proteins. Twenty standard L-amino acids link together through peptide bonds; each has an amino group, a carboxyl group, and a distinct side chain that sets its charge and chemistry.
Peptide: A short chain of amino acids joined by peptide bonds, typically fewer than about 50 residues. Longer chains are called polypeptides or proteins.
Peptide bond: The covalent amide bond that links the carboxyl group of one amino acid to the amino group of the next, releasing one water molecule per bond formed.
Polypeptide: A longer amino-acid chain, usually more than about 50 residues. The line between a long peptide and a small protein is a matter of convention, not a hard rule.
Residue: A single amino acid unit within a peptide chain. It is called a residue because forming each peptide bond removes a water molecule from the free amino acid.
N-terminus: The end of a peptide with a free amino group. Sequences are written and numbered starting from the N-terminus on the left.
C-terminus: The end of a peptide with a free carboxyl group. Many bioactive peptides are amidated here, which changes the charge and mass versus the free acid.
Primary structure: The linear amino-acid sequence of a peptide, which determines every higher-order property from folding to receptor binding.
Disulfide bond: A covalent S-S bridge between two cysteine residues that stabilizes folded structure. It lowers the mass by two hydrogens versus the reduced form and is common in peptides like oxytocin.Oxytocin
Isoelectric point (pI): The pH at which a peptide carries no net electrical charge. It predicts solubility and behavior in ion-exchange chromatography and isoelectric focusing.
Net charge: The sum of positive and negative charges on a peptide at a given pH, set by the ionizable groups (N-terminus, C-terminus, and the side chains of Asp, Glu, His, Cys, Tyr, Lys, Arg).
Molecular weight: The mass of a peptide in daltons (g/mol), summed from its atoms. Average molecular weight uses standard atomic weights; a research vial's true mass can differ if the peptide is modified.
Monoisotopic mass: The mass calculated using the single most abundant isotope of each element. It is the value mass spectrometry reports for small peptides, and it differs slightly from average mass.
Hydrophobicity: How much a residue or peptide repels water. It drives folding, membrane interaction, and how a peptide behaves in reversed-phase chromatography.
GRAVY: Grand Average of Hydropathy: the mean Kyte and Doolittle hydropathy across all residues. Negative values indicate a hydrophilic peptide, positive values a hydrophobic one.
Extinction coefficient: How strongly a peptide absorbs ultraviolet light at 280 nm, driven mostly by tryptophan and tyrosine. It lets researchers estimate concentration from absorbance.
C-terminal amidation: A modification that replaces the C-terminal carboxyl with an amide group. It is common in signaling peptides, increases stability, and shifts the molecular weight versus the free acid.Selank Semax
N-terminal acetylation: Addition of an acetyl group to the N-terminus, which protects against degradation and alters mass. Several research peptides, including some thymosin products, are acetylated.TB-500 (Thymosin Beta-4)Thymosin Alpha-1
Cyclic peptide: A peptide whose chain is closed into a ring, often for stability or receptor selectivity. Bremelanotide (PT-141) is a cyclic melanocortin peptide.PT-141 (Bremelanotide)
L-amino acid: The naturally occurring stereoisomer of amino acids found in human peptides and proteins. D-amino acid substitutions are sometimes used to slow enzymatic breakdown.

Classes & mechanisms

GLP-1: Glucagon-like peptide-1, a gut hormone that boosts insulin, slows gastric emptying, and reduces appetite. Its analogs are the backbone of modern weight-loss medicine.Semaglutide Compare GLP-1 providers, from $139/mo
GLP-1 receptor agonist: A drug that activates the GLP-1 receptor to reduce appetite and improve glucose control. Semaglutide is the best-known example.Semaglutide Liraglutide Compare GLP-1 providers, from $139/mo
GIP: Glucose-dependent insulinotropic polypeptide, a second incretin hormone. Adding GIP activity to GLP-1 is what makes tirzepatide a dual agonist.Tirzepatide Compare GLP-1 providers, from $139/mo
Glucagon receptor: A receptor that raises energy expenditure and fat metabolism when activated. Retatrutide adds glucagon activity on top of GLP-1 and GIP.Retatrutide Compare GLP-1 providers, from $139/mo
Dual agonist: A single molecule that activates two receptors at once. Tirzepatide is a GLP-1 and GIP dual agonist.Tirzepatide Compare GLP-1 providers, from $139/mo
Triple agonist: A molecule that activates three receptors. Retatrutide targets GLP-1, GIP, and glucagon, producing the largest trial weight loss to date.Retatrutide Compare GLP-1 providers, from $139/mo
Incretin: A gut hormone released after eating that stimulates insulin secretion. GLP-1 and GIP are the two principal incretins.Compare GLP-1 providers, from $139/mo
Amylin: A pancreatic hormone that promotes satiety and slows gastric emptying. Cagrilintide is a long-acting amylin analog studied alongside semaglutide.Cagrilintide Pramlintide Compare GLP-1 providers, from $139/mo
Growth hormone (GH): A pituitary hormone that drives growth, repair, and metabolism, largely through IGF-1. Several peptides aim to raise the body's own GH output.Sermorelin Ipamorelin
GHRH: Growth-hormone-releasing hormone, which prompts the pituitary to secrete GH. Sermorelin, CJC-1295, and tesamorelin are GHRH analogs.Sermorelin CJC-1295 Tesamorelin
GHRP: Growth-hormone-releasing peptides, a class that acts on the ghrelin receptor to trigger GH pulses. GHRP-2, GHRP-6, and ipamorelin are examples.GHRP-2 GHRP-6 Ipamorelin
Ghrelin mimetic: A compound that imitates ghrelin at its receptor to stimulate GH release and appetite. MK-677 is an orally active ghrelin mimetic.MK-677 (Ibutamoren)Ipamorelin
Growth hormone secretagogue: Any agent that makes the body secrete its own growth hormone rather than supplying GH directly. Covers both GHRH analogs and ghrelin mimetics.MK-677 (Ibutamoren)Ipamorelin Sermorelin
IGF-1: Insulin-like growth factor 1, the downstream mediator of most of growth hormone's anabolic and repair effects.IGF-1 LR3 IGF-1 DES
Melanocortin: A family of signaling peptides acting on MC receptors to influence pigmentation, appetite, and sexual function. Melanotan and PT-141 are melanocortin analogs.Melanotan II PT-141 (Bremelanotide)
MC4R: The melanocortin-4 receptor, central to appetite regulation and sexual arousal. PT-141 activates it to treat low desire; setmelanotide targets it for rare obesity.PT-141 (Bremelanotide)Setmelanotide
Nootropic peptide: A peptide studied for cognition, focus, or anxiety, often delivered intranasally. Selank and Semax are the archetypes.Selank Semax
Bioregulator: A very short peptide (often two to four residues) hypothesized to modulate gene expression in a specific tissue. Epithalon is the most cited example.Epithalon Pinealon
Angiogenesis: The formation of new blood vessels, a key step in tissue repair. BPC-157 is studied partly for promoting angiogenesis in injured tissue.BPC-157
Collagen synthesis: The production of collagen, the structural protein of skin, tendon, and connective tissue. GHK-Cu and TB-500 are studied for supporting it.GHK-Cu TB-500 (Thymosin Beta-4)
Telomere: The protective cap at the end of a chromosome that shortens with each cell division. Epithalon is studied for its proposed effect on telomerase activity.Epithalon
NAD+: Nicotinamide adenine dinucleotide, a coenzyme central to cellular energy and repair that declines with age. It is delivered by injection, nasal spray, or precursor supplements.NAD+
Mitochondrial peptide: A peptide encoded by or targeting mitochondria to influence energy metabolism and stress resistance. MOTS-c and SS-31 are the leading research examples.MOTS-c SS-31 (Elamipretide)
HPG axis: The hypothalamic-pituitary-gonadal axis that governs reproductive hormones. Kisspeptin and gonadorelin act at its top to trigger LH and FSH release.Kisspeptin Gonadorelin
Myostatin: Also called GDF-8, a protein that limits muscle growth. Blocking it is an active target for preserving lean mass during GLP-1 weight loss.Follistatin 344
Pentadecapeptide: A peptide of fifteen amino acids. BPC-157 is described as a stable gastric pentadecapeptide derived from a protein in gastric juice.BPC-157

Prescription GLP-1

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Pharmacology

Half-life: The time for the plasma concentration of a compound to fall by half. It sets dosing frequency: a one-week half-life supports weekly dosing.
Bioavailability: The fraction of a dose that reaches the bloodstream intact. Injected peptides are near-complete; oral and nasal routes are far lower because peptides are fragile in the gut.
Pharmacokinetics: What the body does to a drug: absorption, distribution, metabolism, and excretion. Abbreviated PK.
Pharmacodynamics: What a drug does to the body: its mechanism, receptor binding, and dose-response relationship. Abbreviated PD.
Steady state: The point at which the amount of a compound entering the body equals the amount cleared, so average concentration stops rising. Reached after roughly five half-lives.
Cmax and Tmax: Cmax is the peak plasma concentration after a dose; Tmax is the time it takes to reach that peak. Together they describe how fast and how high a dose rises.
Receptor agonist: A molecule that binds a receptor and activates it, mimicking the natural signal. Most therapeutic peptides are agonists.
Receptor antagonist: A molecule that binds a receptor and blocks it, preventing the natural signal from acting.
Titration: Gradually increasing a dose over weeks to reach a target while limiting side effects. Standard practice for GLP-1 drugs to reduce nausea.Semaglutide Tirzepatide
Maintenance dose: The steady ongoing dose used after titration to hold an effect, as opposed to the escalating doses used to get there.
Off-label: Use of an approved drug for an indication, dose, or population it was not approved for. Legal for licensed prescribers but outside the label evidence.
Compounded medication: A medication mixed to order by a licensed pharmacy, legal under specific conditions but not FDA-approved and not reviewed before sale. Most telehealth GLP-1 is compounded.Compare GLP-1 providers, from $139/mo
503A pharmacy: A traditional compounding pharmacy that prepares medications for an individual patient prescription under state board oversight.
503B outsourcing facility: A larger compounding facility registered with the FDA that can produce batches without patient-specific prescriptions, under stricter manufacturing standards than a 503A.
Research chemical: A compound sold for laboratory research use only, not for human consumption, with no prescription and no medical oversight. Most research-grade peptide vials fall here.

Sourcing & testing

Certificate of Analysis (COA): A lab document reporting a specific batch's identity and purity. A credible COA is third-party, batch-specific, and shows the actual test method and result.
HPLC purity: Purity measured by high-performance liquid chromatography, reported as a percentage. It is the standard metric buyers look for on a peptide COA.
Mass spectrometry: An analytical method that measures molecular mass to confirm a peptide's identity. Pairing it with HPLC confirms both what a compound is and how pure it is.
Lyophilized: Freeze-dried into a stable powder for shipping and storage. A lyophilized peptide must be reconstituted with liquid before use.
Reconstitution: Dissolving a lyophilized peptide in a diluent, usually bacteriostatic water, to a known concentration. The math determines how many units per dose.
Bacteriostatic water: Sterile water containing 0.9% benzyl alcohol, which inhibits bacterial growth and allows a reconstituted vial to be used across multiple days.
Endotoxin: A bacterial contaminant that causes fever and inflammation if injected. Quality manufacturing tests and controls for low endotoxin levels.
GMP: Good Manufacturing Practice, a set of quality standards for how a product is made, tested, and documented. A meaningful signal only when independently verifiable.
Third-party testing: Analysis performed by an independent lab rather than the seller, which is what makes a purity claim credible rather than self-reported.
FDA-approved vs investigational: FDA-approved means the agency reviewed safety and efficacy for a specific use. Investigational means still in trials; compounded and research-grade peptides are neither approved nor reviewed before sale.
Phase 1/2/3 trial: The staged human testing of a drug: Phase 1 for safety, Phase 2 for early efficacy and dosing, Phase 3 for large confirmatory evidence used for approval.
WADA: The World Anti-Doping Agency, whose prohibited list includes many growth-hormone and IGF-1 peptides. Relevant to any tested athlete.

Dosing & administration

Subcutaneous (SubQ): Injection into the fatty layer just under the skin, the most common route for research peptides and GLP-1 drugs, usually with a small insulin syringe.
Intramuscular (IM): Injection into a muscle, used for some compounds that call for deeper delivery or larger volumes.
Microgram vs milligram: A milligram (mg) is 1,000 micrograms (mcg). Confusing the two is the most common and most dangerous peptide dosing error.
International Unit (IU): A measure of biological activity rather than mass, used for hormones like HCG and HGH. IU does not convert directly to milligrams without a compound-specific factor.HCG (Human Chorionic Gonadotropin)
U-100 insulin syringe: A syringe graduated so that 100 units equals 1 mL. Peptide dosing is usually drawn in these units after reconstitution.
Concentration (mg/mL): The amount of peptide per millilitre of solution after reconstitution. It links the vial size and the water added to the volume drawn per dose.
Microdosing: Using smaller, more frequent doses than a standard schedule, often to smooth side effects or stretch a vial. Common with GLP-1 and GH-axis peptides.
Stack: Using two or more compounds together for complementary effects, such as CJC-1295 with ipamorelin for growth-hormone release.CJC-1295 Ipamorelin
Cycle: A defined on-then-off period of use, intended to manage receptor sensitivity or side effects over time.

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