Forty peptide and biochemistry terms defined in plain English. Bookmark this page if you are new to peptide protocols. Every term you will encounter on Peptide AI is defined here.
A compound that binds a receptor and activates it, mimicking the natural signal. Semaglutide is a GLP-1 receptor agonist.
Read the full entry →The chemical building blocks of peptides and proteins. There are 20 standard amino acids encoded by the human genome.
Read the full entry →A severe systemic allergic reaction. Rare with peptides but possible with any injectable. Symptoms include difficulty breathing, swelling, hives. Seek emergency care immediately.
A compound that binds a receptor without activating it, blocking the natural signal.
A protein on the surface of LDL and other atherogenic particles. ApoB is a more accurate cardiovascular risk marker than LDL cholesterol alone. Tracked by Peptide AI's lab parser.
Read the full entry →Sterile water mixed with 0.9% benzyl alcohol, an additive that suppresses bacterial growth in a punctured multi-dose vial. The standard diluent for reconstituting lyophilized peptides. See our full BAC water breakdown.
Read the full entry →Bioavailability is the share of a dose that reaches the blood. Semaglutide is 89 percent by injection and only 0.4 to 1 percent as the Rybelsus tablet.
Read the full entry →A document from a peptide manufacturer or third-party lab confirming the identity, purity, and mass of a peptide batch. Common purity metric is HPLC purity above 98%.
Read the full entry →What phase 1, 2, 3 and 4 trials test, how many people each enrols, and how many drugs move on, with retatrutide and the GLP-1 drugs as worked examples.
Read the full entry →A drug prepared by a licensed compounding pharmacy for a specific patient, often when the brand version is in shortage. Compounded Semaglutide became widely available during the 2024 Ozempic shortage. See our compounding writeup.
Read the full entry →A confidence interval is the range of values consistent with a trial result. How to read the 95 percent CI in STEP 1, SELECT and SURPASS-CVOT.
Read the full entry →A defined period of using a peptide followed by a defined period off. Common research-context cycles range from 4 to 12 weeks on, 2 to 8 weeks off.
A modification added to CJC-1295 that binds the peptide to serum albumin, extending its half-life from about 30 minutes to about 6 to 8 days.
A DEXA scan splits the body into fat, lean tissue and bone with a very low X-ray dose. What lean mass includes and what GLP-1 DEXA studies found.
Read the full entry →Endotoxin is a fragment of bacterial cell walls that survives sterilisation and can cause fever. How the LAL test works and the 5 EU/kg limit for injectables.
Read the full entry →An estimand defines exactly what a trial result estimates. Why STEP 1 reports 14.9 and 16.9 percent, and SURMOUNT-1 20.9 and 22.5 percent, for one arm.
Read the full entry →Authorization by the US Food and Drug Administration to market a drug for a specific indication. Semaglutide, Tirzepatide, and Tesamorelin are FDA approved; BPC-157, TB-500, Ipamorelin are not.
A hypothalamic hormone that signals the pituitary gland to release growth hormone. GHRH analogs include Sermorelin, Tesamorelin, and CJC-1295.
A second gut hormone, also released after eating, that supports insulin secretion. Tirzepatide and Retatrutide activate both GLP-1 and GIP receptors.
A naturally occurring gut hormone released after eating. It signals satiety, slows gastric emptying, and triggers insulin release. GLP-1 receptor agonists like Semaglutide and Tirzepatide mimic this hormone.
A compound that prompts the pituitary gland to release a natural pulse of growth hormone. Includes Ipamorelin, GHRP-2, GHRP-6, and Hexarelin.
The time it takes the body to clear half a dose. A peptide with a 165 hour half-life (Semaglutide) loses half its blood concentration every 7 days, which supports once-weekly dosing. Use the half-life calculator to model steady state.
Read the full entry →A hazard ratio compares event rates over time between two groups. What SELECT's HR 0.80 means, how it differs from absolute risk, and how to read the interval.
Read the full entry →A biomarker that reflects average blood glucose over the previous 2 to 3 months. A common downstream marker tracked on GLP-1 protocols.
A laboratory technique used to measure peptide purity. A typical Certificate of Analysis reports HPLC purity as a percentage.
How purity is tested →Beat-to-beat variation in heart rate, measured in milliseconds. Higher HRV generally reflects better autonomic recovery. Tracked by Apple Watch, Oura, Whoop, and read into the Peptide AI app via Apple Health and Health Connect.
Read the full entry →High-sensitivity C-reactive protein. A blood marker of systemic inflammation. Elevated hs-CRP is associated with cardiovascular risk.
Read the full entry →A protein produced mostly by the liver in response to growth hormone. IGF-1 mediates most of the systemic effects of GH and is the standard biomarker for tracking GH-axis activity.
Insulin resistance means cells respond weakly to insulin, so more is needed. How HOMA-IR is calculated from fasting glucose and insulin, and GLP-1 data.
Read the full entry →A small disposable syringe marked in insulin units, where 100 units equals 1 mL. Sold in 1 mL (100u), 0.5 mL (50u), and 0.3 mL (30u) capacities. See how to read insulin syringe units.
An injection placed into muscle tissue. Less common for peptides; used for some oil-suspended formulations.
A lump of fat tissue that forms at injection sites that have been used repeatedly. Avoided by rotating injection sites.
Read the full entry →A period at the start of a protocol where doses are higher or more frequent to build tissue concentration. Common with BPC-157 and TB-500.
Freeze-dried. Most research peptides ship as a lyophilized powder in a sealed vial because the dry form is shelf-stable for years at room temperature.
Read the full entry →The lower steady dose used after the loading phase, typically once or twice weekly.
A laboratory technique that confirms the molecular weight of a peptide, used alongside HPLC on a Certificate of Analysis to verify identity.
How purity is tested →Use of an FDA-approved drug for a purpose not listed on its official labeling. Legal for clinicians to prescribe; common in longevity and metabolic medicine.
Read the full entry →A short chain of amino acids, typically 2 to 50 amino acids long. Longer chains are called proteins.
A peptide bond joins one amino acid to the next. How it forms, why enzymes cut it, and how semaglutide and liraglutide were engineered to survive the cutting.
Read the full entry →Peptides come as acetate, TFA or hydrochloride salts, or as the free base. Why a milligram of salt is not a milligram of peptide, and the FDA's view.
Read the full entry →How a drug affects the body. The relationship between a dose and its biological effect.
How a drug moves through the body: absorption, distribution, metabolism, excretion. Half-life, peak concentration, and steady state are PK terms.
Read the full entry →A pea-sized gland at the base of the brain that controls growth, metabolism, and reproduction. Many peptides act on the pituitary to release growth hormone.
A placebo-controlled trial compares a drug with an inactive dummy. Why randomisation and blinding matter, with STEP 1 numbers from the placebo arm.
Read the full entry →A protein on a cell surface or inside a cell that binds a specific molecule (like a peptide) and triggers a downstream biological response.
Read the full entry →The process of mixing a lyophilized peptide with bacteriostatic water to create an injectable solution at a known concentration. Use the free reconstitution calculator.
A compound sold for laboratory research, not human consumption. Most peptides sold online (BPC-157, TB-500, Ipamorelin) are labeled this way and are not FDA approved for human use.
Read the full entry →Two or more peptides used in the same protocol period. Common research stacks include CJC-1295 with Ipamorelin (GH pulse) or BPC-157 with TB-500 (recovery).
The point at which the amount of peptide entering the body and the amount being cleared are roughly equal. Reached after approximately 4 to 5 half-lives at a constant dose.
Read the full entry →An injection placed into the fat layer just under the skin rather than into a vein or muscle. The standard route for most peptide injections.
Read the full entry →A surrogate endpoint is a marker like HbA1c or LDL used in place of outcomes like heart attacks. When they work, when they misled, and GLP-1 data.
Read the full entry →Gradually increasing a dose over weeks instead of starting at the target dose. Common for GLP-1 receptor agonists to reduce gastrointestinal side effects. Use the titration schedule generator.
Read the full entry →The Peptide AI app handles the math, the reminders, and the trend lines. Free on iOS and Android.