A beginner's glossary

Terms are grouped by where you will meet them. For incretin physiology, receptor terms, half-life arithmetic and trial design words, use the encyclopedia's glossary on GLP-1s Explained; this list stops where that one starts.

The medicine

GLP-1. Glucagon-like peptide-1, a gut hormone released after eating that raises insulin when glucose is high, slows stomach emptying and reduces appetite. The medicines copy it.

GIP. Glucose-dependent insulinotropic polypeptide, a second gut hormone. Tirzepatide acts on the receptors for both GIP and GLP-1 (Zepbound section 12.1).

Receptor agonist. A molecule that switches a receptor on the way the natural hormone does. "GLP-1 receptor agonist" or "GLP-1 RA" is the class name.

Semaglutide. The molecule in Ozempic, Wegovy and Rybelsus. Tirzepatide. The molecule in Mounjaro and Zepbound.

Half-life. The time for the amount of drug in the blood to fall by half. About a week for semaglutide, about five days for tirzepatide (section 12.3). Why dosing is weekly.

Steady state. The point where each weekly dose only replaces what was cleared, so the level stops climbing. Four to five weeks for semaglutide, four for tirzepatide (section 12.3).

Subcutaneous. Under the skin, into the fat layer, not into muscle or a vein. All four products are subcutaneous injections.

The label

Label, or prescribing information. The FDA-approved document describing a drug's indications, dosing, warnings, side effects and trial results, organised in numbered sections. Free on DailyMed. Every citation on this site that says "section 5" means a section of this document.

Indication. What the drug is approved for (section 1). Off-label. Prescribed for something not in section 1; legal, at the prescriber's judgment.

Contraindication. A situation in which the label says the drug must not be used (section 4). For all four products: personal or family history of medullary thyroid carcinoma or MEN 2, or a prior serious allergic reaction.

Boxed warning. The most serious class of FDA warning, printed in a box at the top of the label. Here, thyroid C-cell tumors seen in rodents.

MTC. Medullary thyroid carcinoma. MEN 2. Multiple Endocrine Neoplasia syndrome type 2, an inherited condition. Both are contraindications.

Medication Guide. The patient-facing leaflet at the end of the label, written in plain language. The when to call page is built from it.

Adverse reaction. The label's term for a side effect observed in trials, reported as a percentage against placebo (section 6.1).

Dosing

Initiation dose. The first dose on the label ladder (0.25 mg semaglutide, 2.5 mg tirzepatide), used for four weeks to reduce gut reactions. Not a treatment dose (section 2.1).

Titration, or escalation. Increasing the dose in steps at least four weeks apart until a maintenance dose is reached.

Maintenance dose. The dose you stay on. Wegovy 2.4 mg (or 1.7 mg if 2.4 is not tolerated); Zepbound 5, 10 or 15 mg (section 2.1).

Pen. A prefilled injection device. Wegovy and Zepbound pens are single-dose; the Ozempic pen holds several doses and you dial each one.

Vial. A small bottle you draw from with a syringe. Zepbound is sold in single-dose vials; compounded products usually come in multi-dose vials.

Units. The marks on an insulin syringe. 100 units equals 1 mL on a U-100 syringe. Units are a volume, not a dose; the dose in milligrams depends on the vial's concentration. The commonest source of error with compounded products.

Concentration. Milligrams of drug per millilitre of liquid, printed on a vial. Different for every pharmacy and fill.

Brand and compounded

Brand. The FDA-approved product made by the approval holder: Novo Nordisk for semaglutide, Eli Lilly for tirzepatide.

Compounded. Made by a pharmacy for an individual patient under section 503A of the Food, Drug and Cosmetic Act, or by an outsourcing facility under 503B. Not FDA approved; FDA does not review compounded drugs for safety, effectiveness or quality before sale. Not interchangeable with brand products.

Interchangeable. A specific FDA term meaning a product may be substituted for another at the pharmacy. No compounded GLP-1 has that status; this site uses "not interchangeable" in that sense.

Certificate of analysis (COA). A pharmacy or lab document reporting what a batch was tested for. The Compounding Explained site explains how to read one.

Population and money

BMI. Body-mass index: weight in kilograms divided by height in metres squared. The labels use 30 for obesity and 27 for overweight (see who the labels say they are for).

Weight-related comorbid condition. The label's phrase for a condition that goes with excess weight; the examples given are hypertension, dyslipidemia, type 2 diabetes, and in the Zepbound label obstructive sleep apnea and cardiovascular disease.

A1c. Haemoglobin A1c, a blood test reflecting average glucose over about three months. The main measure in the diabetes labels; the Lab Guide covers it.

Prior authorization (PA). An insurer's requirement that the prescriber document you meet its criteria before it pays. Formulary. The insurer's list of covered drugs and their tiers. Step therapy. A requirement to try a cheaper option first.

Savings card. A manufacturer program that lowers the copay for people with commercial insurance; not valid with Medicare or Medicaid. Self-pay. Paying the published cash price, for example through LillyDirect or NovoCare Pharmacy.

Responder. Trial language for someone who reached a threshold, usually 5% weight loss. "Responder rate" is the share who did.