GLP-1 glossary: the terms on the labels, in the trials and in the pharmacy, defined with their sources

Definitions are short on purpose. Each points at where the term lives, so you can read the original. Label section numbers refer to the current Zepbound and Wegovy prescribing information on DailyMed unless stated; Mounjaro and Ozempic use the same structure with slightly different numbering.

Drugs and devices

GLP-1 receptor agonist. A drug that activates the receptor for glucagon-like peptide-1, a gut hormone released after eating that increases insulin release, reduces glucagon, slows gastric emptying and reduces appetite. Semaglutide is one. Label section 12.1.

GIP. Glucose-dependent insulinotropic polypeptide, a second gut hormone. Tirzepatide activates both the GIP and GLP-1 receptors (label 12.1), which is why it is sometimes called a dual agonist rather than a GLP-1.

Semaglutide. The active ingredient in Wegovy, Ozempic and Rybelsus. Elimination half-life about 1 week; present in the circulation for about 5 to 7 weeks after the last dose (Wegovy 12.3).

Tirzepatide. The active ingredient in Zepbound and Mounjaro. Elimination half-life about 5 to 6 days (Zepbound 12.3).

Wegovy. Novo Nordisk's semaglutide for chronic weight management, cardiovascular risk reduction and MASH; injection up to 2.4 mg weekly (a 7.2 mg regimen is also on the label), plus a tablet form.

Ozempic. Novo Nordisk's semaglutide for type 2 diabetes; up to 2 mg weekly in a multi-dose pen.

Zepbound. Lilly's tirzepatide for weight management and obstructive sleep apnoea; 2.5 to 15 mg weekly.

Mounjaro. Lilly's tirzepatide for type 2 diabetes; same doses and devices as Zepbound.

Single-dose pen. A pre-filled auto-injector delivering one fixed dose, then discarded. Wegovy's has a built-in needle and a yellow progress bar; Zepbound's has a lock ring, two audible clicks and a grey plunger that shows when the dose is complete. Instructions for use.

KwikPen. Lilly's multi-dose pen for Zepbound and Mounjaro: four weekly doses, a new pen needle each time, primed before every injection, discarded 30 days after first use or after four doses (Zepbound 16.2 and instructions for use).

FlexTouch. Novo Nordisk's multi-dose pen for Wegovy: flow check before the first dose, new needle each time, discarded 56 days after first use (Wegovy 16).

Priming. Expelling a small amount of drug from a multi-dose pen before injecting to remove air and confirm flow. Required before every KwikPen injection; Ozempic and Wegovy FlexTouch call it a flow check and require it only with a new pen.

Flow check. See priming.

Single-dose vial. A glass vial containing one dose, drawn up with a syringe. Zepbound's label says to use a syringe able to measure the dose, for example a 1 mL syringe reading 0.5 or 0.6 mL, and a new syringe and needle each time (2.4).

Multi-dose vial. A vial containing several doses; on the Zepbound label, discarded 30 days after first use or after four weekly doses.

U-100 insulin syringe. A syringe graduated in units for insulin at 100 units per mL; 100 units equals 1 mL, 50 units equals 0.5 mL. Widely used with compounded vials, which is where units-versus-millilitre errors arise. Not part of the brand labels.

Pen needle. A screw-on needle for multi-dose pens. The Ozempic label supplies NovoFine Plus 32G 4 mm needles and permits compatible needles up to 8 mm.

Sharps container. An FDA-cleared container, or a heavy-duty plastic container with a screw lid, for used pens, needles and syringes. FDA sharps disposal guidance.

Dosing

Escalation (titration). The stepwise increase from a starting dose to a maintenance dose, to reduce GI adverse reactions. Wegovy: 0.25, 0.5, 1, 1.7, 2.4 mg at 4-week intervals (2.2). Zepbound: 2.5 mg for 4 weeks, then 2.5 mg increments after at least 4 weeks per step (2.1).

Starting (initiation) dose. The first step: 0.25 mg for Wegovy and Ozempic, 2.5 mg for Zepbound and Mounjaro. The Zepbound label says 2.5 mg is not approved as a maintenance dose.

Maintenance dose. The dose intended for long-term use. Wegovy 2.4 mg (1.7 mg if 2.4 is not tolerated); Zepbound 5, 10 or 15 mg.

Delayed escalation. The Wegovy label's option to wait a further 4 weeks before the next step if a dose is not tolerated (2.2).

Missed dose. Zepbound and Mounjaro: take within 4 days (96 hours), otherwise skip. Wegovy: take if the next scheduled dose is more than 2 days (48 hours) away, otherwise skip; after two or more consecutive missed doses, reinitiate escalation at a lower dose. Ozempic: take within 5 days, otherwise skip. Never two doses to catch up.

Changing the injection day. Permitted if the gap between doses is at least 72 hours (Zepbound, Mounjaro) or 48 hours (Wegovy, Ozempic).

Subcutaneous. Into the fat layer under the skin, not into muscle or a vein. All four labels.

Injection sites. Abdomen, thigh, upper arm, per every label; rotate with each dose. Instructions for use add: 2 inches from the navel, back of the upper arm with a helper, avoid tender, bruised, red, hard, scarred or stretch-marked skin.

Site rotation. Changing the injection spot every dose. Labels require it; the Zepbound vial instructions give the reason (lipodystrophy and localised cutaneous amyloidosis). See the rotation guide.

Lipohypertrophy. Firm thickening of subcutaneous fat from repeated injection at the same spot; erratic absorption results. FIT recommendations (Frid 2016, PubMed 27594187).

Lipodystrophy. The label's umbrella word for pitting or thickening of skin at injection sites (Zepbound vial instructions).

Localised cutaneous amyloidosis. Protein lumps at repeatedly injected sites, named on the Zepbound vial instructions as a reason to rotate.

Injection site reaction. Redness, itching or swelling at the site; 6 to 8 percent on Zepbound versus 2 percent on placebo (6.1).

Storage

Cold chain. Refrigeration at 2 to 8 C (36 to 46 F) before first use, all products (section 16).

Room-temperature allowance. Time permitted out of the fridge, not above 30 C (86 F): Zepbound and Mounjaro single-dose 21 days total; Wegovy single-dose pen 28 days before cap removal; Ozempic and Wegovy FlexTouch 56 days after first use. See the storage guide.

Temperature excursion. Any time outside the labelled range. Above 30 C or frozen: no label allowance.

In-use date. The date a multi-dose pen or vial was first used, from which its discard clock runs.

Beyond-use date (BUD). The compounding equivalent of an expiry date, set by the pharmacy under compounding standards and generally shorter than a manufacturer's expiry (FDA compounding Q&A).

Adverse effects and warnings

Boxed warning. The label's most prominent warning; for all four, the risk of thyroid C-cell tumours seen in rodents, with a contraindication in people with a personal or family history of medullary thyroid carcinoma or MEN 2.

Medullary thyroid carcinoma (MTC). A thyroid cancer of C-cells; the reason for the boxed warning and the contraindication.

MEN 2. Multiple Endocrine Neoplasia syndrome type 2, an inherited condition including MTC; a contraindication.

Pancreatitis. Inflammation of the pancreas; severe persistent abdominal pain, sometimes to the back. Label warning: discontinue if suspected, do not restart if confirmed.

Acute gallbladder disease. Gallstones and cholecystitis; label warning. Wegovy label: cholelithiasis 1.6 percent versus 0.7 percent placebo in adults.

Acute kidney injury due to volume depletion. Kidney damage after fluid loss from GI effects; label warning in all four. See the hydration guide.

Hypoglycaemia. Low blood glucose; label warning for use with insulin or sulfonylureas.

Pulmonary aspiration. Stomach contents entering the lungs under general anaesthesia or deep sedation; label warning in all four, instruction to inform clinicians of planned procedures. See the surgery guide.

Gastroparesis. Severely delayed stomach emptying; the labels say the drugs are not recommended in severe gastroparesis.

Ileus. A stopped bowel; listed in the postmarketing sections of the Wegovy and Zepbound labels with intestinal obstruction and severe constipation.

Diabetic retinopathy complications. Worsening eye disease with rapid glucose improvement; label warning in people with type 2 diabetes.

Hypersensitivity. Allergic reactions including anaphylaxis and angioedema; a prior serious reaction is a contraindication.

Telogen effluvium. Diffuse temporary hair shedding after a physiological stress such as rapid weight loss; the pattern behind the labels' hair loss entries (Wegovy 3 percent, Zepbound 4 to 5 percent). See the hair guide.

Trials

STEP. The semaglutide 2.4 mg weight-management programme. STEP 1 (PubMed 33567185): 14.9 percent loss at 68 weeks. STEP 4 (PubMed 33755728): withdrawal design. STEP 1 extension (PubMed 35441470): regain a year off drug.

SURMOUNT. The tirzepatide weight-management programme. SURMOUNT-1 (PubMed 35658024): 15.0, 19.5 and 20.9 percent at 5, 10 and 15 mg over 72 weeks. SURMOUNT-4 (PubMed 38078870): withdrawal design. SURMOUNT-5 (PubMed 40353578): head to head with semaglutide.

SURPASS. The tirzepatide type 2 diabetes programme. SURPASS-2 (PubMed 34170647) compared it with semaglutide 1 mg.

SELECT. The semaglutide cardiovascular outcomes trial in people with obesity and established cardiovascular disease, the basis of Wegovy's March 2024 cardiovascular indication.

Randomised withdrawal. A trial design in which everyone takes the drug, then some are switched to placebo without knowing; SURMOUNT-4 and STEP 4. See the stopping guide.

DXA substudy. A subset of trial participants scanned by dual-energy X-ray absorptiometry to measure fat and lean mass; STEP 1 and SURMOUNT-1 both had one. See the muscle guide.

Lean mass. Everything that is not fat or bone on a DXA scan: muscle, organs, water. Larger than muscle alone.

Regulation and pharmacy

Prescribing information (label). The FDA-approved document describing a drug's indications, dosing, warnings and data. Hosted on DailyMed; revised several times a year.

Instructions for use (IFU). The patient-facing device instructions in the box, part of the labelling.

Indication. The condition a drug is approved to treat, in section 1 of the label. Insurance criteria are keyed to it. See the insurance guide.

Off-label. Use for a purpose or in a manner not in the label. Legal for a prescriber; usually not covered by insurance.

503A pharmacy. A state-licensed compounding pharmacy that prepares medication for an individual patient on a prescription. FDA human drug compounding page.

503B outsourcing facility. An FDA-registered facility that compounds in bulk under manufacturing standards.

Compounded drug. A medication prepared by a pharmacy rather than a manufacturer. Not FDA approved for safety, effectiveness or quality; not interchangeable with the brand product. FDA compounding pages. Compounded semaglutide and tirzepatide are what FormBlends' pharmacies dispense, as described on its semaglutide and tirzepatide pages.

Certificate of analysis (COA). A laboratory report on a specific batch of a compounded or bulk drug stating what was tested and found. The compounding site explains how to read one.

Prior authorisation. An insurer's requirement that criteria be documented before a drug is covered.

Step therapy. An insurer's requirement to try a specified drug first.

Pregnancy exposure registry. A voluntary study collecting outcomes from pregnancies exposed to a drug; phone numbers in section 8.1 of the Wegovy and Zepbound labels.

Multi-society perioperative guidance. The 2024 joint statement from five anaesthesia, gastroenterology and surgical societies on managing GLP-1 users around procedures (Kindel 2024, PubMed 39482213).