Agonist
A compound that binds a receptor and activates it, mimicking the natural signal. Semaglutide is a GLP-1 receptor agonist.
Maintenance phase
The lower steady dose used after the loading phase, typically once or twice weekly.
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.
The chemical building blocks of peptides and proteins. There are 20 standard amino acids encoded by the human genome.
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.
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.
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%.
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 crackdown writeup.
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.
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 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 hypothalamic hormone that signals the pituitary gland to release growth hormone. GHRH analogs include Sermorelin, Tesamorelin, and CJC-1295.
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.
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.
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.
High-sensitivity C-reactive protein. A blood marker of systemic inflammation. Elevated hs-CRP is associated with cardiovascular risk.
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.
An injection placed into muscle tissue. Less common for peptides; used for some oil-suspended formulations.
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.
A lump of fat tissue that forms at injection sites that have been used repeatedly. Avoided by rotating injection sites.
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.
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.
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.
A short chain of amino acids, typically 2 to 50 amino acids long. Longer chains are called proteins.
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.
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 protein on a cell surface or inside a cell that binds a specific molecule (like a peptide) and triggers a downstream biological response.
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.
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.
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.
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.
The Peptide AI app handles the math, the reminders, and the trend lines. Free on iOS and Android.