GLP-1 Glossary

Summary: A working vocabulary for reading about GLP-1 medications. The terms below come up across the site and in clinical trial papers, FDA labels, and patient instructions.

This content is for informational purposes only and is not medical advice. Always consult a qualified healthcare provider before starting, changing, or stopping any medication.

GLP-1 (glucagon-like peptide-1): A hormone released by L-cells in the small intestine within minutes of eating. It triggers insulin release from the pancreas when blood glucose is elevated, slows gastric emptying, suppresses glucagon, and signals fullness to the brain. The natural hormone breaks down within about two minutes, which is why drugmakers had to engineer longer-lasting versions to make it useful as a medication.
GLP-1 receptor agonist: A drug that binds to and activates the GLP-1 receptor, mimicking the natural hormone but designed to last hours or days instead of minutes. Semaglutide (Ozempic, Wegovy, Rybelsus), liraglutide (Victoza, Saxenda), and dulaglutide (Trulicity) are pure GLP-1 agonists. Tirzepatide (Mounjaro, Zepbound) is a dual GIP/GLP-1 agonist. Retatrutide is an investigational triple GIP/GLP-1/glucagon agonist.
Incretin: A category of gut hormones that boost insulin secretion in response to food. The two main human incretins are GLP-1 and GIP. They explain the "incretin effect," the observation that the same glucose load given orally produces a much larger insulin response than when given intravenously. Incretin signaling is blunted in type 2 diabetes, which is part of why GLP-1 receptor agonists work so well for that condition.
GIP (glucose-dependent insulinotropic polypeptide): The other major incretin hormone. GIP is released from K-cells in the upper small intestine. Its role is more complex than GLP-1's: it amplifies insulin output, but it also has effects on fat tissue and energy balance. Tirzepatide activates the GIP receptor in addition to the GLP-1 receptor, and that dual action is one explanation for its larger weight loss numbers.
HbA1c (glycated hemoglobin): A blood test that reflects average blood glucose over the prior two to three months. It is the standard measure of glycemic control in type 2 diabetes. A1c is reported as a percentage, with treatment goals usually below 7 percent. GLP-1 medications typically reduce A1c by 1.0 to 2.6 percentage points depending on the drug, dose, and starting value.
Titration: The practice of starting at a low, sub-therapeutic dose and stepping up at fixed intervals (usually every four weeks for weekly GLP-1 medications) to allow the body to tolerate the drug. The starting doses do not produce much weight loss or glycemic benefit on their own; they exist to soften the nausea, vomiting, and other gastrointestinal side effects that come with rapid escalation.
Subcutaneous (SC): An injection placed into the layer of fatty tissue just beneath the skin, rather than into a muscle or vein. GLP-1 injections are subcutaneous and go into the abdomen, the front of the thigh, or the back of the upper arm. The needle is short (typically 4 to 8 millimeters) and the technique is straightforward enough that most patients self-inject at home.
Gastroparesis: Delayed emptying of the stomach. GLP-1 medications slow gastric emptying as part of their normal mechanism, which is partly why people feel full faster. In some patients this slowing is severe enough to cause persistent nausea, vomiting of undigested food, or, in rare cases, complications during anesthesia. Anesthesiologists now routinely ask whether patients are on a GLP-1 before scheduled surgery.
Compounded medication: A medication custom-prepared by a licensed pharmacy from raw active pharmaceutical ingredient, rather than purchased as a finished FDA-approved product. Compounded semaglutide and tirzepatide became widely available during 2023 and 2024 while the brand drugs were on the FDA shortage list. Compounded versions are not FDA-approved as products, which means batch testing, sterility, and consistency depend on the individual pharmacy's standards.
503A and 503B pharmacies: Two categories of compounding pharmacy under US law. A 503A pharmacy compounds patient-specific prescriptions, one prescription at a time, after a clinician's order. A 503B "outsourcing facility" compounds in larger batches under tighter quality rules and registers with the FDA. Most telehealth-prescribed GLP-1 compounding flows through 503A pharmacies.
Off-label use: Prescribing an FDA-approved medication for a use that is not on the official label. Using Ozempic for weight loss in a non-diabetic patient is off-label, since Ozempic is approved only for type 2 diabetes; Wegovy is the version of semaglutide approved for chronic weight management. Off-label prescribing is legal and common but is not always covered by insurance.
Boxed warning (black box): The strongest safety warning the FDA can require on a drug label. All GLP-1 receptor agonists carry a boxed warning about thyroid C-cell tumors, based on findings in rats. The warning translates to a contraindication for patients with a personal or family history of medullary thyroid carcinoma or multiple endocrine neoplasia type 2.
MACE (major adverse cardiovascular events): A composite trial endpoint that combines cardiovascular death, non-fatal heart attack, and non-fatal stroke into a single outcome. Cardiovascular outcome trials such as SUSTAIN-6 (semaglutide), LEADER (liraglutide), and SELECT (semaglutide in non-diabetic obesity) used MACE as their primary measure of benefit.
Lean mass: The non-fat portion of body weight, primarily skeletal muscle, organ tissue, and bone. Any rapid weight loss costs lean mass, and GLP-1 medications are no exception. Body composition substudies of STEP and SURMOUNT trials show lean mass accounts for roughly a quarter to forty percent of weight lost without structured resistance training. Lifting weights and eating adequate protein during the loss phase reduce that share.
Indication: The specific condition a medication is FDA-approved to treat. Ozempic's indications are type 2 diabetes and reducing the risk of major adverse cardiovascular events in adults with both type 2 diabetes and established cardiovascular disease. Wegovy's indications include chronic weight management in qualifying adults and adolescents and reducing cardiovascular risk in adults with obesity or overweight plus established cardiovascular disease.