GLP-1 Glossary: 26 Core Terms, Defined From the FDA Label

**GLP-1.** Glucagon-like peptide-1 is a hormone released from cells in the gut wall after a meal. The Saxenda prescribing information (Novo Nordisk, Section 12.1, via DailyMed, accessed 14 September 2026) describes GLP-1 as a physiological regulator of appetite and calorie intake and states that native GLP-1 has a half-life of 1.5 to 2 minutes because the enzymes DPP-4 and neutral endopeptidase break it down almost immediately. The medications on this site are engineered copies that survive far longer. Our explainer on how GLP-1 medications work covers the mechanism in more depth.

**GIP.** Glucose-dependent insulinotropic polypeptide is a second gut hormone, released from the upper small intestine. The Mounjaro label (Eli Lilly, Section 12.1) names GIP and GLP-1 as the two targets of tirzepatide. GIP has no approved single-hormone medication of its own; it enters this vocabulary only because tirzepatide activates both receptors.

**Incretin.** An incretin is any gut hormone that increases insulin secretion in response to food, in a glucose-dependent way. GLP-1 and GIP are the two incretins that matter clinically. Nauck and Meier (Diabetes, Obesity and Metabolism, 2018) describe the incretin effect as a two- to three-fold higher insulin response to glucose taken by mouth than to the same glucose given intravenously, which is the physiological reason a gut hormone can influence blood sugar.

**Receptor agonist.** A receptor is a protein on a cell surface; an agonist is a molecule that binds to it and switches it on. The Ozempic label (Section 12.1) states that semaglutide selectively binds to and activates the GLP-1 receptor, the target for native GLP-1. That sentence is the whole definition: the drug imitates the hormone at the receptor. An antagonist would block the receptor instead; none of the medications on this site are antagonists.

**Gastric emptying.** Gastric emptying is the rate at which the stomach passes its contents to the small intestine. The Ozempic label (Section 12.2) states that semaglutide causes a delay of early postprandial gastric emptying, and the Mounjaro label (Section 12.2) states that tirzepatide delays gastric emptying, that the delay is largest after the first dose, and that the effect diminishes over time. Delayed gastric emptying is commonly described as contributing to fullness and to some of the gastrointestinal effects listed in our overview of GLP-1 side effects; the labels themselves state the delay without assigning it those effects.

**Semaglutide.** Semaglutide is a GLP-1 analogue made by Novo Nordisk and sold as Ozempic, Wegovy and Rybelsus in the United States. The Ozempic label (Section 12.1, via DailyMed, accessed 14 September 2026) states it has 94% sequence homology to human GLP-1 and that its long half-life comes principally from albumin binding, which reduces kidney clearance and protects the molecule from being broken down. The same section notes it is stabilised against the DPP-4 enzyme that destroys native GLP-1 within minutes. Our Ozempic reference page quotes the full pharmacokinetic section.

**Tirzepatide.** Tirzepatide is Eli Lilly's molecule, sold as Mounjaro and Zepbound. The Mounjaro label (Section 12.1) defines it as a GIP receptor and GLP-1 receptor agonist that contains a C20 fatty diacid enabling albumin binding, which prolongs the half-life. It is the only marketed medication in this glossary that acts on two receptors. The two brands share a label chemistry but carry different approved indications; our Mounjaro reference page and Zepbound reference page keep the labels' figures separate.

**Liraglutide.** Liraglutide is the older, once-daily GLP-1 receptor agonist from Novo Nordisk, sold as Victoza and Saxenda. The Saxenda label (Section 12.1) states 97% amino acid sequence homology with human GLP-1 and a plasma half-life of 13 hours after subcutaneous injection, which is why it is dosed daily rather than weekly. Since 23 December 2024 it has also existed as an FDA-approved generic; that date and the word generic are defined further down this page. Our Saxenda reference page has the label detail.

**Half-life.** The elimination half-life is the time it takes for the concentration of a drug in the blood to fall by half. It is a population average, not a personal measurement. The Ozempic label (Section 12.3, via DailyMed, accessed 14 September 2026) states an elimination half-life of approximately 1 week for semaglutide and adds that the molecule will be present in the circulation for about 5 weeks after the last dose. The Mounjaro label states approximately 5 days for tirzepatide, the Zepbound label approximately 5 to 6 days, and the Saxenda label 13 hours for liraglutide.

**Time to peak (Tmax).** Tmax is the time after a dose at which blood concentration reaches its maximum. Labels give it as a window, not a point: the Ozempic label states maximum concentration is reached 1 to 3 days post dose, and the Mounjaro label states a range of 8 to 72 hours, which the Zepbound label summarises as a median of 24 hours. Any single hour figure you see in a table or an app is a midpoint someone chose from inside that window.

**Steady state.** Steady state is the point at which repeated dosing and elimination balance out, so that levels stop climbing from one dose to the next. The Ozempic label states steady-state exposure is achieved following 4 to 5 weeks of once-weekly administration; the Mounjaro label states steady-state plasma concentrations were achieved following 4 weeks. This is the pharmacological reason the first month on a weekly medication feels different from the third.

**Bioavailability.** Absolute bioavailability is the fraction of an administered dose that reaches the bloodstream unchanged, measured against an intravenous reference. The Ozempic label states 89% for injected semaglutide; the Mounjaro label states a mean of 80% for tirzepatide; the Saxenda label states approximately 55% for liraglutide. The Wegovy label (Section 12.3) estimates approximately 1% to 2% for the oral semaglutide tablet, which is why the tablet and the injection can never be compared by their printed strengths. Our oral Wegovy reference page explains that gap.

The practical use of these four terms is modest. GLP-1 Tracker's medication level estimator draws an educational curve from the doses you log and a labeled half-life of the kind quoted here; it is an illustration of accumulation and decay, never a blood level, and it changes nothing about the schedule your prescriber set.

**Titration.** Titration is the stepwise adjustment of a dose over time, set by the prescriber. For this class the direction is upward from a starting level, and the labels state the reason: the Zepbound prescribing information (Eli Lilly, Section 2.1, via DailyMed, accessed 14 September 2026) instructs prescribers to follow its escalation schedule to reduce the risk of gastrointestinal adverse reactions. This glossary does not reproduce any schedule, interval or strength. What a person can do is record the step they were given and when it changed; how to record a GLP-1 dose change shows the log entry.

**Maintenance dose.** A maintenance dose is the level at which titration stops and treatment continues long term. The same Zepbound section states that prescribers should consider treatment response and tolerability when selecting the maintenance dosage, and that a lower maintenance dosage may be considered if a patient does not tolerate one. In other words, maintenance is an individual decision, not a fixed destination, which is why our dosing schedule guide describes the concept and never a number.

**Off-label.** The FDA's patient page on unapproved uses of approved drugs defines off-label use as an approved drug being used for a condition it is not approved to treat, given in a different way, or given in a different dose. The same page states that healthcare providers generally may prescribe a drug for an unapproved use when they judge it medically appropriate, and that in doing so the FDA has not determined the drug is safe and effective for that use. Ozempic prescribed for weight management alone, for example, is off-label, because the Ozempic label's indication is type 2 diabetes.

**Prior authorization.** HealthCare.gov's glossary defines preauthorization, also called prior authorization, prior approval or precertification, as a decision by a health insurer or plan that a service, treatment or prescription drug is medically necessary, and notes it is not a guarantee of coverage. For GLP-1 medications it is the paperwork step between the prescription and the pharmacy. Our scenario on an insurance denial covers what happens when it is refused.

**Prefilled pen.** A prefilled pen is an injection device that arrives already containing the medication, so nothing is drawn up from a vial. Every pen on this site is prefilled; the Saxenda Instructions for Use (via DailyMed, accessed 14 September 2026) describe that device as a prefilled, dial-a-dose, single-patient-use pen.

**Single-dose pen versus multi-dose pen.** A single-dose pen holds one dose and is discarded after it. The Wegovy label (Section 3) lists a single-dose pen, and the Mounjaro and Zepbound labels list a single-dose pen and a single-dose vial. A multi-dose pen holds several doses; the Ozempic label describes single-patient-use pens with multiple weekly doses, the Wegovy FlexTouch pen is labeled as delivering four doses, and the Mounjaro and Zepbound KwikPens are labeled as containing four doses each. Single-patient-use means the pen is never shared, even with a fresh needle. Disposal is a separate question, answered in what to do with used GLP-1 pens.

**Autoinjector.** An autoinjector is a pen whose dose is preset and whose needle stays hidden; the injection fires when the device is pressed against the skin. The Wegovy single-dose pen Instructions for Use state that the dose is already set, that the needle will not be seen, and that the injection starts when the needle cover is pressed firmly against the skin. That is an autoinjector in everything but name. Dial-a-dose pens such as Ozempic and Saxenda are not autoinjectors: the user selects the dose with a dial and presses a button.

**Compounded.** A compounded drug is one mixed or altered by a pharmacy or outsourcing facility rather than manufactured under an approved application. The FDA's page Compounding and the FDA: Questions and Answers states that compounded drugs are not FDA-approved and that the agency does not verify their safety, effectiveness or quality before they are marketed. Our guide to compounded semaglutide goes into the practical differences.

**Generic.** The FDA's Generic Drugs: Questions and Answers page defines a generic as a medication created to be the same as an already marketed brand-name drug in dosage form, safety, strength, route of administration, quality, performance characteristics and intended use, and states that a generic must be shown to be bioequivalent, delivering the same amount of active ingredient at the same rate and extent. The FDA's press announcement of 23 December 2024 records the first generic referencing Victoza (liraglutide injection), made by Hikma. No FDA-approved generic semaglutide or tirzepatide existed on the access date of this page.

**Biosimilar.** A biosimilar is a copy of a biologic, a product made from living cells or microorganisms. The FDA's biosimilars pages define it as a biologic that is highly similar to, and has no clinically meaningful differences from, an approved reference product. Semaglutide, tirzepatide and liraglutide are regulated as drugs rather than biologics, because the FDA's definition of a biological protein in 21 CFR 600.3(h)(6) covers only amino acid polymers longer than 40 amino acids and all three are shorter; the first liraglutide copy was approved through the generic (ANDA) pathway on 23 December 2024, so their copies are generics, not biosimilars. The word appears in this glossary only because it is so often misused for them.

**BMI.** Body mass index is weight in kilograms divided by the square of height in metres. The U.S. Centers for Disease Control and Prevention (adult BMI categories page, accessed 14 September 2026) groups adults as underweight below 18.5, healthy weight from 18.5 to below 25, overweight from 25 to below 30, and obesity at 30 or greater, and calls BMI a screening measure to be considered alongside other factors. The Wegovy and Zepbound labels describe their trial populations in BMI terms, which is why the number follows people through every prescription. Our BMI calculator does the arithmetic.

**Lean mass.** Lean mass, or lean body mass, is everything in the body that is not fat: muscle, bone, organs and water. It matters on a GLP-1 because weight lost is never fat alone, and the share that is lean mass is what protein intake and resistance training are meant to protect. Our guide to GLP-1s and muscle loss discusses what the trials measured and what a scale cannot.

**Telogen effluvium.** Telogen effluvium is diffuse, non-scarring hair shedding that follows a physiological stressor, including rapid weight loss. The StatPearls entry on the NCBI Bookshelf (National Library of Medicine) describes a causative event typically about 3 months before shedding begins, within a range of 1 to 6 months, and describes the acute form as self-limited and spontaneously reversible. The Zepbound label (Section 6.1) states that hair loss in treated patients was associated with weight reduction and was reported in 7.1% of female versus 0.5% of male patients on Zepbound, against 1.3% and 0% on placebo; the Wegovy label lists hair loss among its adverse reactions. Neither label uses the clinical term, which is why it is defined here.

**Hypoglycemia.** Hypoglycemia is blood glucose below the normal range. Each label in this class carries a warning about it: the Ozempic label's Section 5.5 concerns hypoglycemia with concomitant use of insulin secretagogues or insulin, and the Wegovy label's Section 5.4 and the Zepbound label's Section 5.7 state that the medication lowers blood glucose and can cause hypoglycemia, with the risk increased when combined with insulin or an insulin secretagogue such as a sulfonylurea. The threshold values and the monitoring instructions belong to the label and the prescriber, not to a glossary. What a person can usefully log is timing: when a symptom occurred relative to the dose, the meal and any other medication, which is the record GLP-1 Tracker is built to hold and a prescriber can actually use.