What a GLP-1 medicine is, in plain words

You have probably heard the brand names before you heard what they do. This page goes the other way: the mechanism first, in ordinary words, then the names.

The hormone

After you eat, cells in your gut release a small protein called glucagon-like peptide-1GLP-1A gut hormone released after eating that raises insulin, slows the stomach and reduces appetite; the medicines copy it and last much longer., GLP-1 for short. It does several things at once. It tells the pancreas to release insulin when blood sugar is high. It slows the rate at which your stomach empties. And it acts on the parts of the brain that decide whether you feel hungry or full. Your own GLP-1 is broken down within minutes, so its effect on any one meal is short.

The medicine

Semaglutide is a made version of GLP-1 with two changes: one stops it being broken down quickly, the other lets it stick to a blood protein so it stays in circulation. The result is a molecule that acts like GLP-1 but lasts about a week (Wegovy label section 12.3, half-lifehalf-lifeThe time it takes for the amount of drug in the blood to fall by half; a week-long half-life is what allows once-weekly dosing. about 1 week). That is why it is injected once a week rather than after every meal.

Tirzepatide does the same trick with two hormones. It acts on the GLP-1 receptor and on the receptor for a second gut hormone, GIPGIPA second gut hormone released after eating; tirzepatide activates its receptor as well as the GLP-1 receptor. (glucose-dependent insulinotropic polypeptide). The Zepbound label describes it as a GIP receptor and GLP-1 receptor agonist (section 12.1), with a half-life of about 5 days (section 12.3). Once weekly again.

"AgonistagonistA molecule that switches a receptor on, the way the body's own hormone would." is the word the labels use. It means the molecule switches the receptor on, the way the natural hormone would. You will see "GLP-1 receptor agonist" and "GLP-1 RA" used for the whole class. The beginner glossary has the other terms you will meet.

What you notice

Two effects reach you directly. Appetite falls, and food stays in your stomach longer, so you feel full sooner and for longer. Both come from the same receptor activity that produced the trial results. Both also explain the most common side effects, which are gut side effects: nausea, constipation, diarrhea, vomiting (Wegovy label section 6.1; Zepbound label section 6.1). The week-by-week pages cover when those tend to show up and settle.

The blood sugar effect is glucose dependent: the insulin signal only fires when glucose is high, so on its own a GLP-1 rarely drives sugar too low. The labels flag a higher risk of low blood sugarhypoglycemiaBlood sugar falling too low; uncommon with these medicines alone but more likely alongside insulin or a sulfonylurea. when the medicine is combined with insulin or a sulfonylureasulfonylureaAn older class of diabetes tablet that pushes the pancreas to release insulin; combining it with a GLP-1 raises the risk of low blood sugar. (Wegovy section 5.4; Zepbound section 5.4), which is a question for your prescriber if you take either.

The four names

Same molecule, different label, different maximum dose, sometimes a different device. The label population page turns the "for" column into the BMIBMIWeight divided by height squared; a rough screening number that the labels use to describe who the medicines were studied in. and diagnosis wording the labels actually use.

Compounded versions

You will also see semaglutide and tirzepatide sold as compoundedcompoundedMade by a pharmacy from raw ingredients for a prescription rather than by the brand manufacturer; not FDA approved and not interchangeable with the brand product. products, made by a pharmacy rather than by Novo Nordisk or Lilly. FDA does not review compounded drugs for safety, effectiveness or quality before they are sold; they are not FDA approved, and they are not interchangeable with the brand products (FDA, Human Drug Compounding). FormBlends, which runs this site, sells compounded semaglutide and compounded tirzepatide. The brand versus compounded page sets out what that difference means for a beginner, in one place.

If you want the deep version

This site stays at the level a first-time reader needs. For the receptor pharmacology, the approval history with dates, per-label dosing tables and the other agents in the class (liraglutide, dulaglutide, the oral tablets, the molecules still in trials), read the network's encyclopedia, GLP-1s Explained, starting with how the class works.