Peptide glossary
The vocabulary you need to read a peptide page without guessing — receptors, evidence terms, measurement units and the differences that matter most.
- Agonist
- A molecule that binds a receptor and activates it, producing the same kind of signal the body's own messenger would. GLP-1 receptor agonists are drugs shaped closely enough to GLP-1 to switch its receptor on.
- Amylin analogue
- A synthetic version of amylin, a hormone co-secreted with insulin that slows gastric emptying and signals satiety. Cagrilintide is the analogue most often paired with GLP-1 agonists in trials.
- Antagonist
- A molecule that binds a receptor and blocks it rather than activating it, preventing the natural messenger from signalling.
- Bacteriostatic waterBAC water
- Sterile water containing roughly 0.9% benzyl alcohol as a preservative, which inhibits bacterial growth so a multi-dose vial can be entered more than once. Distinct from sterile water for injection, which has no preservative. Benzyl alcohol preparations are contraindicated in neonates under FDA labelling.
- Bioavailability
- The fraction of an administered amount that reaches the bloodstream unchanged. Injected peptides are near-complete; oral peptides are typically a fraction of one percent without an absorption enhancer.
- Concentration
- How much drug sits in each millilitre of solution, written mg/mL or mcg/mL. After reconstitution it equals the vial's mass divided by the diluent volume added.
- Diluent
- The liquid added to a lyophilised (freeze-dried) vial to dissolve its contents. Bacteriostatic water and sterile water for injection are the two common diluents.
- DPP-4Dipeptidyl peptidase-4
- An enzyme that rapidly degrades native GLP-1 and GIP, giving natural GLP-1 a half-life of a couple of minutes. Long-acting analogues are engineered to resist it.
- Dual agonist
- A single molecule that activates two receptors. Tirzepatide, a GIP and GLP-1 receptor agonist, is the best-known example.
- Endotoxin
- Bacterial cell-wall fragments that cause fever and inflammation even when no live bacteria remain. Pharmaceutical manufacturing tests for them; research-grade material frequently does not.
- Excipient
- A non-active ingredient in a vial — buffer, bulking agent, stabiliser — present to protect or dissolve the active compound.
- GHSGrowth hormone secretagogue
- A compound that prompts the pituitary to release its own growth hormone rather than supplying growth hormone directly. Includes GHRH analogues such as sermorelin and tesamorelin, and ghrelin-receptor agonists such as ipamorelin and hexarelin.
- GIPGlucose-dependent insulinotropic polypeptide
- An incretin hormone released from the upper small intestine after eating. Adding GIP receptor activity to GLP-1 activity is the basis of tirzepatide.
- GLP-1Glucagon-like peptide-1
- An incretin hormone cleaved from proglucagon in the gut that amplifies glucose-dependent insulin release, suppresses glucagon, slows gastric emptying and reduces appetite through central pathways.
- GLP-2Glucagon-like peptide-2
- A sibling peptide from the same proglucagon precursor that acts on a different receptor to promote intestinal growth and absorption. Its analogues treat short bowel syndrome, not obesity.
- GLP-3
- Not a real hormone or receptor. An informal online nickname for retatrutide, an investigational triple agonist of the GLP-1, GIP and glucagon receptors.
- Glucagon receptor
- The receptor for glucagon, chiefly on liver cells. Activating it raises hepatic glucose output and energy expenditure and mobilises liver fat, which is why it is included in triple agonists.
- Half-life
- The time for the amount of drug in the body to fall by half. It sets how often a compound would need to be given to maintain exposure, and how long it lingers after stopping.
- Incretin
- A gut hormone released in response to food that increases insulin secretion more than the same glucose given intravenously would. GLP-1 and GIP are the two human incretins.
- IUInternational unit
- A measure of biological activity, defined separately for each substance against a WHO reference standard. IU is not a mass and never converts to milligrams by a universal factor — and it is not the same thing as the 'units' printed on an insulin syringe.
- Lyophilised
- Freeze-dried. Peptides are usually supplied as a lyophilised powder or cake because they are far more stable dry than in solution.
- Melanocortin receptor
- A family of receptors (MC1R through MC5R) involved in pigmentation, appetite, inflammation and sexual arousal. Targeted by peptides such as bremelanotide and melanotan II.
- Peptide
- A short chain of amino acids — conventionally up to around fifty — joined by peptide bonds. Longer chains are called proteins. Most peptides are digested if swallowed, which is why they are typically injected.
- PharmacodynamicsPD
- What the drug does to the body: which receptors it hits and what effect follows.
- PharmacokineticsPK
- What the body does to the drug: absorption, distribution, metabolism and elimination, summarised in figures like half-life and peak concentration.
- Phase 1 / 2 / 3
- Stages of human trials. Phase 1 tests safety and pharmacokinetics in a small group; phase 2 tests activity and dose-finding; phase 3 tests efficacy and safety at scale against a comparator. Approval normally follows successful phase 3.
- Placebo-subtracted
- A result reported after removing the change seen in the placebo arm, isolating the effect attributable to the drug rather than to trial participation and lifestyle support.
- Proglucagon
- The precursor protein that is cleaved differently in the pancreas and gut to yield glucagon, GLP-1, GLP-2 and other fragments.
- Receptor
- A protein on or in a cell that a signalling molecule binds to, triggering a response inside the cell. Which receptors a peptide binds is the single most useful fact about it.
- Reconstitution
- Dissolving a lyophilised powder by adding a measured volume of diluent. It changes concentration, never the total amount of drug in the vial.
- Research chemical
- Material sold for laboratory use only. It is not manufactured, tested or released to pharmaceutical standards, and identity, purity, endotoxin load and sterility are frequently unverified.
- SubcutaneousSC / SubQ
- Into the fat layer beneath the skin — the usual route for peptide injections, giving slower, steadier absorption than intramuscular or intravenous routes.
- Syringe units
- The numbers printed on an insulin syringe barrel. They are volume markings tied to insulin concentration: on a U-100 syringe, 100 units equal 1 mL, so one unit is 0.01 mL. They are not IU of a peptide and not milligrams.
- Titration
- Stepwise adjustment of an amount over time, used clinically to improve tolerability. GLPWiki does not publish titration schedules.
- Triple agonist
- A molecule that activates three receptors at once. Retatrutide targets the GLP-1, GIP and glucagon receptors and remains investigational.
- U-100 / U-50 / U-40
- Insulin syringe scales. The number is units per millilitre: a U-100 syringe reads 100 units to the millilitre, U-50 reads 50, U-40 reads 40. Using the wrong scale changes what a given mark means.
EDUCATIONAL REFERENCE ONLY · Not medical advice. Nothing here diagnoses, treats, cures or prevents any disease, and nothing here is a dosing recommendation. Consult a licensed clinician before any treatment decision.