GLP-1, “GLP-2” & “GLP-3”: What People Really Mean by Semaglutide, Tirzepatide & Retatrutide
They sound like three generations of the same thing. The real science is a little different — and much more interesting.
If you’ve spent any time in the weight-loss, wellness or peptide world lately, you’ve probably heard people talk about GLP-1, GLP-2 and GLP-3.
Usually, they’re referring to three compounds:
“GLP-1” → Semaglutide
“GLP-2” → Tirzepatide
“GLP-3” → Retatrutide
It’s an incredibly easy way to remember the progression.
One receptor. Two receptors. Three receptors.
There’s just one problem:
That’s not actually what GLP-1, GLP-2 and GLP-3 mean scientifically.
So before we compare the three compounds everyone is talking about, we need to clear up one of the biggest naming misconceptions in the current weight-loss conversation.
What People Think GLP-1, GLP-2 and GLP-3 Mean
Online, the terminology has essentially become shorthand.
When someone says “GLP-1,” they’re often talking about semaglutide.
When someone says “GLP-2,” they’re often talking about tirzepatide.
And increasingly, when someone says “GLP-3,” they’re talking about retatrutide.
You can see how we got here.
Semaglutide targets one major metabolic receptor: GLP-1.
Tirzepatide targets two: GLP-1 + GIP.
Retatrutide targets three: GLP-1 + GIP + glucagon.
So social media turned that into:
GLP-1 → GLP-2 → GLP-3.
Simple? Absolutely.
Scientifically accurate? Not quite.
Here’s what these compounds actually are:
| Common online nickname | Compound people usually mean | What it actually targets |
|---|---|---|
| “GLP-1” | Semaglutide | GLP-1 |
| “GLP-2” | Tirzepatide | GLP-1 + GIP |
| “GLP-3” | Retatrutide | GLP-1 + GIP + glucagon |
Semaglutide really is a GLP-1 receptor agonist. Tirzepatide is a dual GIP/GLP-1 receptor agonist. Retatrutide is an investigational triple GIP/GLP-1/glucagon receptor agonist.
So let’s look at what adding those extra receptors actually means.
Semaglutide: The One People Call “GLP-1”
This is the easiest one because the nickname is essentially correct.
Semaglutide is a GLP-1 receptor agonist.
GLP-1 — glucagon-like peptide-1 — is a hormone naturally produced in the body.
Among other things, GLP-1 signaling is involved in glucose regulation, insulin secretion, appetite and food intake.
Semaglutide was designed to activate that same receptor.
For weight management, semaglutide is marketed as Wegovy; semaglutide is also used in other FDA-approved products for different indications. The FDA identifies semaglutide as a GLP-1 receptor agonist.
One of the effects that made this class of medication so significant is its influence on appetite.
For many people, that changed the weight-loss equation.
Instead of constantly fighting hunger, food cravings and thoughts about the next meal, medications targeting GLP-1 could influence some of the biological signals driving those behaviors.
And then researchers began asking:
What if we target another metabolic pathway at the same time?
That’s where tirzepatide enters the story.
Tirzepatide: What People Are Calling “GLP-2”
Here’s where the internet terminology starts getting messy.
Tirzepatide is sometimes called “GLP-2.”
But tirzepatide doesn’t actually target the GLP-2 receptor.
Instead, tirzepatide activates:
GLP-1 + GIP
GIP stands for glucose-dependent insulinotropic polypeptide, another naturally occurring hormone involved in metabolic regulation.
That makes tirzepatide a dual GIP and GLP-1 receptor agonist. FDA labeling specifically identifies activity at both receptors.
Tirzepatide is the active ingredient in Mounjaro, approved for type 2 diabetes, and Zepbound, approved for chronic weight management and other indications.
So why does everyone call it “GLP-2”?
Most likely because it’s an incredibly convenient nickname.
Semaglutide hits one receptor.
Tirzepatide hits two.
Therefore, people started saying GLP-1 and GLP-2.
But there’s an especially important reason that terminology isn’t technically correct.
GLP-2 Is Already a Real Thing
This is the part that surprises a lot of people.
GLP-2 actually exists.
It stands for glucagon-like peptide-2, and it’s a naturally occurring hormone with its own receptor and its own biological functions, particularly involving the gastrointestinal tract.
So calling tirzepatide a “GLP-2” doesn’t mean it’s the second generation of GLP-1.
And tirzepatide isn’t primarily working through the actual GLP-2 system.
When people use “GLP-2” to describe tirzepatide, what they generally mean is:
It’s the one that targets TWO receptors.
The scientifically useful term is dual agonist.
And that brings us to the compound generating an enormous amount of attention right now.
Retatrutide: What People Are Calling “GLP-3”
Retatrutide is increasingly being called “GLP-3.”
Again, the nickname is based on receptor count.
Where semaglutide targets one receptor and tirzepatide targets two, retatrutide targets three different metabolic receptors:
GLP-1
GIP
Glucagon
That’s why its proper description is a triple hormone receptor agonist.
Even Lilly, the company developing retatrutide, has addressed the “GLP-3” nickname directly. Lilly says the term is scientifically inaccurate and arose as informal shorthand because retatrutide targets three receptors.
There isn’t a newly discovered “GLP-3 receptor” hiding inside the body.
The third receptor being added is the glucagon receptor.
And that’s an important distinction.
Why Add the Glucagon Receptor?
At first, glucagon can seem like an unusual addition.
GLP-1 and GIP are incretin hormones with established roles in metabolic signaling.
Glucagon is different.
It’s deeply involved in the body’s management of energy and glucose, and glucagon-receptor signaling can influence energy expenditure and fuel metabolism.
The idea behind retatrutide is therefore more sophisticated than simply making GLP-1 stronger.
Researchers are investigating what happens when GLP-1, GIP and glucagon receptor activity are combined within a single molecule.
That’s what makes retatrutide particularly interesting.
And the clinical results have attracted plenty of attention.
Retatrutide Is NOT FDA Approved Yet
This deserves its own section because the distinction is extremely important.
As of September 2026, retatrutide remains an investigational drug.
It is not currently FDA approved, and Lilly says it has not been approved by any regulatory agency. Retatrutide is being studied in a large Phase 3 clinical development program covering obesity, type 2 diabetes and several obesity-related health conditions.
Several Phase 3 trials have now produced positive topline results.
That’s exciting.
But successful clinical trials and FDA approval are not the same thing.
Lilly announced in July 2026 that it now has the clinical data package to support regulatory submissions for several potential retatrutide indications. The company currently plans to submit retatrutide to the FDA for U.S. approval in the first quarter of 2027.
After submission, the FDA still has to review the application and determine whether the evidence supports approval.
So although you’ll hear plenty of speculation that retatrutide is “about to be FDA approved,” there is currently no confirmed approval date.
The more accurate description is:
Retatrutide is approaching the regulatory submission stage after multiple positive Phase 3 trials.
And until an approval actually occurs, it remains investigational.
The FDA also currently warns that retatrutide is not an ingredient in any FDA-approved drug and cannot legally be used in compounding under federal law.
So What’s the Actual Progression?
This is where everything clicks.
Forget:
GLP-1 → GLP-2 → GLP-3
Instead, think:
SINGLE → DUAL → TRIPLE
Semaglutide
GLP-1
↓
Single receptor agonist
Tirzepatide
GLP-1 + GIP
↓
Dual receptor agonist
Retatrutide
GLP-1 + GIP + glucagon
↓
Triple receptor agonist
That is the evolution researchers are actually exploring.
It’s not the discovery of GLP-1, followed by GLP-2, followed by GLP-3.
It’s the development of molecules capable of activating multiple metabolic signaling pathways simultaneously.
Does Three Receptors Mean Retatrutide Is Automatically “Better”?
This is another place where social media tends to oversimplify things.
It’s very tempting to assume:
One receptor = good.
Two receptors = better.
Three receptors = best.
But biology isn’t an iPhone upgrade.
Adding another receptor changes the mechanism of the drug. It doesn’t automatically establish that the drug will be the best option for every person.
Effectiveness is only one part of the equation.
Researchers also have to evaluate tolerability, side effects, dosing, long-term safety, treatment discontinuation and how different patient populations respond.
That’s exactly why drugs go through large Phase 3 trials and regulatory review before they’re approved.
Retatrutide’s Phase 3 results make the triple-agonist approach particularly interesting.
They don’t eliminate the need for that process.
And What About Actual GLP-2?
Still there.
Still a real hormone.
Still having absolutely nothing to do with being “version two” of Ozempic.
And somewhere, presumably, very annoyed that TikTok stole its name.
Actual GLP-2 is a distinct hormone involved heavily in intestinal physiology.
That’s why there’s an important difference between saying:
“GLP-2”
and
“a drug that targets two receptors.”
Those aren’t the same thing.
Why This Naming Confusion Actually Matters
You could argue that they’re just nicknames.
And in casual conversation, that’s true.
If someone says “GLP-3” and everyone in the conversation knows they’re talking about retatrutide, communication technically happened.
But terminology starts mattering when people make decisions about their health based on what they’re hearing online.
Calling tirzepatide “GLP-2” can make people believe it targets an actual GLP-2 pathway.
Calling retatrutide “GLP-3” can make it sound like it’s simply the third, newest and therefore superior version of an established medication class.
Neither interpretation is accurate.
The real mechanisms are actually much more interesting.
The Compound Edit Cheat Sheet
If you remember nothing else from this article, remember this:
SEMAGLUTIDE — “GLP-1”
This nickname works reasonably well. Semaglutide is an FDA-approved GLP-1 receptor agonist.
TIRZEPATIDE — “GLP-2”
Common internet shorthand, but scientifically inaccurate. Tirzepatide is an FDA-approved dual GIP + GLP-1 receptor agonist.
RETATRUTIDE — “GLP-3”
Another internet nickname, not a scientific classification. Retatrutide is an investigational triple GIP + GLP-1 + glucagon receptor agonist and is not currently FDA approved. Lilly plans to submit it for U.S. approval in Q1 2027.
The Compound Edit Takeaway
Semaglutide, tirzepatide and retatrutide aren’t simply versions 1, 2 and 3 of the same medication.
They’re part of a much more interesting evolution in metabolic research.
Semaglutide showed what targeting GLP-1 could accomplish.
Tirzepatide combined GLP-1 with GIP.
Retatrutide is investigating what happens when GLP-1, GIP and glucagon receptor activity are combined in one molecule.
So yes — when someone online says GLP-1, GLP-2 and GLP-3, there’s a good chance they’re talking about semaglutide, tirzepatide and retatrutide.
Now you know what they actually mean.
And more importantly, you know what each compound actually is.
Continue Exploring
At The Compound Edit, we dig into the science behind peptides, metabolic health, performance and longevity — including the difference between what researchers are actually studying and what social media says they’re studying.
The Compound Edit is affiliated with Chuk BioWorks. That relationship helps support this publication, while our goal remains to explore emerging research honestly — including where the evidence is strong, where it’s still developing and where the internet may be getting a little ahead of the science.
EXPLORE CHUK BIOWORKS → www.chukbioworks.com
The Compound Edit is for educational and informational purposes only and is not medical advice. Some compounds discussed may be investigational or not approved for human use. Always discuss medical treatment decisions with a qualified healthcare professional.
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