You've probably seen "GLP-1" everywhere: on news sites, in podcasts, in your doctor's waiting room. And if you've been paying attention, you've probably also heard people say things like "Ozempic is a GLP-1, Mounjaro is a GLP-2, and Retatrutide is a GLP-3."
That framing makes intuitive sense as a tidy progression: first generation, second generation, third generation. The problem is, it's not accurate, and the confusion matters because the actual categories tell you something important about how these drugs work.
What is a GLP, anyway?
GLP stands for Glucagon-Like Peptide. Glucagon-like peptides are hormones your body produces naturally. They're released by your gut after you eat and play a role in regulating blood sugar, appetite, and digestion.
The number after "GLP" refers to which peptide it is, not which generation of drug or how many receptor targets it has.
- GLP-1 is a hormone released primarily from your small intestine after eating. It stimulates insulin release, slows digestion, and reduces appetite signals in the brain. It became the basis of a drug class because synthetic versions turned out to be highly effective for weight loss and blood sugar management.
- GLP-2 is a different hormone from the same region of the gut. Its primary job is intestinal health: it helps maintain and repair the lining of the small intestine. It has nothing to do with weight loss. (Teduglutide, a GLP-2 receptor agonist, is approved for short bowel syndrome, not obesity.)
- GLP-3 doesn't have a firmly established role in the same way. It appears in some research literature but is not a recognized drug class the way GLP-1 and GLP-2 are.
Where do the famous drugs fit?
Semaglutide (Ozempic, Wegovy, Rybelsus) is a GLP-1 receptor agonist. It mimics GLP-1, the appetite-and-insulin hormone, by binding to and activating the GLP-1 receptor. Calling semaglutide a "GLP-1 drug" is accurate: GLP-1 is the hormone it mimics.
Tirzepatide (Mounjaro, Zepbound) is often called a "GLP-2" in casual conversation. This is incorrect. Tirzepatide is a dual GLP-1/GIP receptor agonist. GIP stands for Glucose-Dependent Insulinotropic Polypeptide, a different gut hormone entirely, unrelated to GLP-2. Tirzepatide targets both the GLP-1 receptor and the GIP receptor simultaneously. The reason it tends to outperform semaglutide in trials isn't that it's a "second-generation GLP": it's that it activates an additional receptor pathway that semaglutide doesn't touch. The "GLP-2" label conflates the drug's informal numbering with GLP-2, the gut hormone involved in intestinal repair.
Retatrutide is in late-stage clinical trials and showing striking early results. It's been casually called a "GLP-3," but this label isn't accurate either. Retatrutide is a triple receptor agonist: it targets GLP-1, GIP, and glucagon receptors. The glucagon piece is what's new. Adding glucagon receptor activation appears to boost metabolic rate and fat breakdown beyond what the dual-agonist approach captures. The informal "third" label comes from "more receptors means a higher number," not from any actual connection to the GLP-3 peptide.
Why the confusion happens
The mix-up is understandable. Here's the pattern that creates it:
- Semaglutide was the first of the class to become widely known. People started calling it "the GLP-1 drug."
- Tirzepatide arrived second, with dual receptor action. "GLP-2" followed by informal analogy.
- Retatrutide is arriving third, with triple receptor action. "GLP-3" followed the same pattern.
It's a clean, memorable shorthand that makes intuitive sense. It just happens to shadow an existing scientific naming system that means something completely different. The actual GLP-1, GLP-2, and GLP-3 hormones are numbered as distinct biological molecules. The drugs are named after their receptor targets, not after a generational ladder.
Drugs and their actual receptor targets
| Drug | Brand Names | Receptor Targets |
|---|---|---|
| Semaglutide | Ozempic, Wegovy, Rybelsus | GLP-1 only |
| Tirzepatide | Mounjaro, Zepbound | GLP-1 + GIP |
| Retatrutide | (in trials) | GLP-1 + GIP + Glucagon |
| Teduglutide | Gattex | GLP-2 only (intestinal repair) |
What this means for choosing between them
The honest answer is that mechanism matters less than outcome for most people, and individual responses vary significantly.
Semaglutide is well-studied, widely used, and effective for many people. Its mechanism is the most targeted of the three.
Tirzepatide appears to produce greater average weight loss in head-to-head comparisons, likely because the dual mechanism provides additive effects that semaglutide's single-receptor approach doesn't match.
Retatrutide is showing early results that are striking, but it isn't yet approved. The triple mechanism raises real questions about side effect profiles at higher doses, including muscle loss, that researchers are still working through.
For most people today, the conversation with a prescriber will center on their specific health profile, tolerance for side effects, and cost and insurance access, not which numbered bucket the drug falls into. The terminology matters more as the class expands: understanding that GLP-1, GLP-2, and GLP-3 describe biology means you can read new research accurately when it appears, rather than defaulting to a mental model that doesn't match what the papers actually say.
Where to go from here
For a deeper look at how GLP-1 receptor agonists work, what the major trials showed, and what the honest caveats are, the GLP-1s explainer covers the science in more detail.
If you're navigating vendor listings and seeing "GLP-2" or "GLP-3" on product pages, the plain-language post on how vendors name these compounds explains what those labels typically mean in a research context and how to verify what you're actually looking at.