GLP-3 and Retatrutide: Why People Call It That
Retatrutide is sometimes informally called "GLP-3" online because it acts on three receptors (GLP-1, GIP, and glucagon), though this is not an official medical term. The nickname caught on across Reddit, TikTok, and weight loss forums as a shorthand way to distinguish retatrutide from "GLP-1" drugs like semaglutide and the "GLP-1/GIP" combination in tirzepatide. While the label is catchy and intuitive, it is technically inaccurate in ways that are worth understanding if you want to speak about these medications with confidence.
Where the Nickname Came From
The progression in obesity pharmacology has followed a clear numerical pattern that makes the "GLP-3" label feel almost inevitable. Semaglutide was the breakout GLP-1 receptor agonist, targeting a single incretin receptor. People called it a "GLP-1 drug." Tirzepatide came next, targeting both GLP-1 and GIP receptors. People naturally started calling it a "dual agonist" or, in informal circles, the "GLP-2" (which is also technically wrong, but we will get to that). Then retatrutide arrived with three receptor targets, and "GLP-3" followed as the logical next step in the informal naming sequence.
The nickname works as a communication tool because it instantly conveys the key selling point: this medication does more than its predecessors. It targets three things instead of two or one. In a social media environment where brevity wins, "GLP-3" communicates more information in fewer characters than "triple receptor agonist targeting GLP-1, GIP, and glucagon receptors." The trade-off is accuracy, but for casual conversation, most people are willing to make that trade.
Why "GLP-3" Is Technically Wrong
The term "GLP-3" implies that there is a receptor or hormone called GLP-3 that retatrutide activates. There is not. The "GLP" in GLP-1 stands for glucagon-like peptide, a specific family of hormones produced in the gut. There are exactly two members of this family: GLP-1 and GLP-2. There is no GLP-3 in human biology.
GLP-1 is the well-known incretin hormone that stimulates insulin secretion, suppresses glucagon release (paradoxically, given retatrutide's mechanism), slows gastric emptying, and reduces appetite. GLP-2 is a related peptide that primarily promotes intestinal growth and nutrient absorption. It is the basis for teduglutide (Gattex), a medication used for short bowel syndrome. GLP-2 has nothing to do with obesity treatment.
Retatrutide's third receptor target is the glucagon receptor, not a hypothetical "GLP-3" receptor. Glucagon itself is actually the parent hormone from which the GLP family gets its name (glucagon-like peptides are so named because they share structural similarity with glucagon). But glucagon and GLP-1 are distinct hormones with distinct receptors and distinct biological effects.
So the accurate description of retatrutide is a "GLP-1/GIP/glucagon triple receptor agonist." It targets three different receptors from two related but separate hormone families: the incretin family (GLP-1 and GIP) and glucagon itself.
The Actual Receptor Breakdown
To be precise about what retatrutide does, here is what each of its three receptor targets contributes:
GLP-1 receptor. This is the foundation of modern obesity pharmacology. Activation reduces appetite through central nervous system effects, slows gastric emptying to promote satiety, improves insulin secretion in response to meals, and has cardiovascular benefits. This is the receptor that semaglutide, liraglutide, and other "-glutide" medications target. Retatrutide's GLP-1 activity provides the core appetite suppression and metabolic benefits shared by the entire class.
GIP receptor. Glucose-dependent insulinotropic polypeptide (GIP) is the other major incretin hormone. Its role in obesity treatment is more nuanced and was initially counterintuitive to researchers, as GIP was historically associated with fat storage. However, pharmacological activation of the GIP receptor at the doses used in medications like tirzepatide and retatrutide appears to enhance weight loss, improve insulin sensitivity, and contribute to better fat distribution. The GIP receptor is what distinguishes tirzepatide from semaglutide, and its inclusion in retatrutide carries those same benefits forward.
Glucagon receptor. This is retatrutide's unique addition. Glucagon receptor activation increases hepatic fat oxidation (the liver burns stored fat for energy), boosts thermogenesis (the body generates more heat, burning more calories at rest), and promotes lipolysis in adipose tissue (breaking down stored body fat). The glucagon receptor is why retatrutide produced that remarkable 86% reduction in liver fat and may explain why its total weight loss exceeded tirzepatide's despite sharing the GLP-1 and GIP components.
Why the Naming Confusion Matters
In casual conversation, calling retatrutide "GLP-3" is harmless shorthand. But the inaccuracy can cause real confusion in several contexts.
Medical conversations. If you tell your doctor you are interested in "the GLP-3 drug," they may not immediately know what you mean, or they may need to correct the terminology before they can have a productive conversation with you. Using the correct name, retatrutide, or even "Lilly's triple agonist," will communicate more effectively with healthcare providers.
Research and reading. If you search for "GLP-3" in medical databases like PubMed, you will not find retatrutide. You will find some unrelated research papers and possibly papers about GLP-2 that discuss the naming conventions of the glucagon-like peptide family. Searching for "retatrutide" or "LY3437943" will get you the actual clinical data you want.
Understanding the mechanism. Thinking of retatrutide as "GLP-3" implies that it simply has "more GLP activity" than its predecessors, when in reality its third target is a completely different type of receptor with completely different biological effects. The glucagon receptor does things that GLP-1 and GIP cannot do. It is not just more of the same. It is a qualitatively different mechanism that adds fat burning and energy expenditure to the equation in ways that pure incretin agonism does not.
The Evolution of Obesity Medications in Context
The progression from one receptor to two to three reflects a broader trend in obesity medicine toward more comprehensive metabolic intervention. Each generation of medication has addressed a wider set of the biological drivers of obesity.
First-generation GLP-1 agonists like liraglutide (Saxenda) produced modest weight loss of 5-8% by reducing appetite. Second-generation GLP-1 agonists like semaglutide (Wegovy) achieved 15-17% weight loss with more potent formulations and optimized dosing. The addition of GIP in tirzepatide pushed results to 20-22%. And now retatrutide, with the addition of the glucagon receptor, has shown 24% weight loss in Phase 2 data with continued weight loss trajectories that had not yet plateaued.
Each step in this progression represents not just a numerical increase in weight loss but a deeper engagement with the metabolic pathways that regulate body weight. The "GLP-3" nickname, while inaccurate, captures the public's intuitive understanding that each new medication represents a step up in comprehensiveness and effectiveness.
What Comes After Three Receptors?
If the trend continues, researchers are already exploring what a "quad agonist" or even more complex multi-receptor medication might look like. Amylin receptor agonists (the mechanism behind cagrilintide, which Novo Nordisk is developing in combination with semaglutide as CagriSema) represent another pathway being added to the mix. Some early-stage research is looking at combining incretin agonism with other targets like the FGF21 pathway or melanocortin-4 receptor.
The future of obesity pharmacology is likely to involve increasingly sophisticated combinations of receptor targets, each selected to address a specific aspect of energy regulation. Whether the informal naming will progress to "GLP-4" and "GLP-5" remains to be seen, but the underlying pharmacology will only get more complex and more effective.
Using the Right Terms
You do not need to be a pharmacologist to talk about retatrutide accurately. Here is a simple framework: if you are chatting online and someone mentions "GLP-3," you will know they mean retatrutide. If you are talking to a doctor, use "retatrutide" or describe it as a "triple agonist targeting GLP-1, GIP, and glucagon receptors." And if someone asks what makes it different from semaglutide or tirzepatide, the answer is the glucagon receptor, which directly increases fat burning and energy expenditure in ways the other medications cannot.
The nickname will probably stick around regardless. Language evolves based on utility, not accuracy, and "GLP-3" is undeniably useful shorthand. Just know what it actually means, and you will be ahead of most of the conversation.