Is Retatrutide a GLP-1? GLP-1 vs GLP-2 vs GLP-3 Explained

Yes, partly. Retatrutide activates the GLP-1 receptor, the same target as semaglutide (Wegovy, Ozempic), but the same molecule also activates the GIP and glucagon receptors, so Lilly calls it a triple agonist. If you're asking "is retatrutide a GLP-1?" because you've seen it called "GLP-3", that's a nickname: there's no hormone called GLP-3, and Lilly's own FAQ calls the label scientifically inaccurate.

The GLP-1 part is why retatrutide's side effects look familiar to anyone who has used Zepbound or Wegovy. The glucagon receptor is the one neither of those drugs touches, and it's where most of the open questions sit.

So is retatrutide a GLP-1 agonist?

It is one, with two extra targets. Lilly describes retatrutide as "a single molecule that activates the body's receptors" for GIP, GLP-1 and glucagon. Semaglutide works on one of those three. Tirzepatide (Zepbound, Mounjaro) works on two, GIP and GLP-1. Retatrutide adds glucagon.

The three aren't hit equally. In lab tests on cells, summarized in a 2024 Cell Discovery paper on retatrutide's structure, retatrutide was 8.9 times as potent as natural GIP at the GIP receptor, but only 0.4 times as potent as natural GLP-1 at the GLP-1 receptor and 0.3 times as potent as glucagon at its own receptor. Lilly's discovery paper in Cell Metabolism (2022) describes the same shape: balanced glucagon and GLP-1 activity, with more GIP activity on top. So "a GLP-1" gets the family right and the proportions wrong.

For practical purposes, though, retatrutide counts as a GLP-1 receptor agonist. The Zepbound and Wegovy labels both advise against combining those drugs with any other GLP-1 receptor agonist. That's label guidance for approved drugs, but the logic carries over: adding retatrutide to semaglutide or tirzepatide doubles up on the same receptor, and no trial has tested it.

Reta is also both a peptide and a GLP-1 drug. GLP-1 is itself a peptide hormone, and Lilly's Cell Metabolism paper describes retatrutide as a "triple agonist peptide".

GLP-1 vs GLP-2 vs GLP-3, and which ones are real

GLP-1 and GLP-2 are real hormones, cut from the same precursor protein, proglucagon, which also makes glucagon, according to the NIH's gene summary for GCG. Cells in your gut release both after you eat. GLP-1 raises insulin when blood sugar is high, holds back glucagon, slows the stomach and makes you feel full. GLP-2 works mostly on the gut itself, helping its lining grow and absorb nutrients.

GLP-3 isn't a hormone at all. Proglucagon yields glucagon, GLP-1, GLP-2 and a glicentin-like peptide, and none of them is called GLP-3. Lilly's FAQ says "GLP-3" is "a scientifically inaccurate label used informally in the media" for triple agonists, and suggests "triple agonist" instead. The nickname came from the three receptors.

Retatrutide isn't a GLP-2 drug either: the GLP-2 receptor isn't among the three Lilly lists. The FDA-approved GLP-2 drug we found, teduglutide, is a daily injection for people with short bowel syndrome who depend on IV nutrition.

Retatrutide vs GLP-1 drugs, side by side

This table puts retatrutide's main Phase 3 obesity trial next to the trials behind Zepbound and Wegovy. Retatrutide figures come from Lilly's May 2026 TRIUMPH-1 topline release. The tirzepatide and semaglutide columns are class evidence from their own trials and FDA labels, not retatrutide data.

All three bring the same kind of stomach trouble. Lilly says the side effects in TRIUMPH-1 were "generally consistent with trials of other incretin-based therapies", and nausea at retatrutide's top dose sits right next to semaglutide 2.4 mg on its label. For the brand-by-brand detail, see retatrutide vs Zepbound and retatrutide vs Wegovy.

What the table can't compare

That table holds three trials, and they enrolled different people. TRIUMPH-1 enrolled adults averaging 112.7 kg with a BMI of 40.0, while SURMOUNT-1 started at 104.8 kg and a BMI of 38.0. The trials also ran for different lengths: 80, 72 and 68 weeks.

The counting differed too. Lilly's headline 28.3% is an "efficacy estimand": roughly, what happens if everyone had stayed on the drug. Counted the other way, including people who stopped or switched treatment, 12 mg gave 25.0%. SURMOUNT-1's 20.9% and STEP 1's 14.9% are reported the second way. So the fairer line-up is 25.0% against 20.9% against 14.9%, and it's still three trials, not one.

The direct comparison is TRIUMPH-5, which randomizes about 800 adults with obesity to retatrutide or tirzepatide for 80 weeks; ClinicalTrials.gov lists its primary completion as November 2026, an estimate. Against semaglutide, TRANSCEND-T2D-2 compares the two in type 2 diabetes, with blood sugar (A1C) at 80 weeks as the main measure. Its registry entry shows the main data collection finished in August 2026, but as of September 2026 Lilly hasn't released results from either trial.

Our read: a gap of about 4 points over tirzepatide on the fairer count, across separate trials, points one way but doesn't settle it. If you're weighing retatrutide against Zepbound, TRIUMPH-5 is the number to wait for.

What the glucagon arm adds, and what it costs

Glucagon is best known for raising blood sugar, so putting it in a weight-loss drug sounds odd. Lilly's discovery paper credits the glucagon receptor with raising energy use on top of the appetite effect from GIP and GLP-1. That finding came from work in obese mice, and it hasn't been measured the same way in the published human trials we read.

In people, the clearest signal so far is in the liver. In a 98-person substudy of the Phase 2 trial, published in Nature Medicine in 2024, liver fat fell by 82.4% on 12 mg at 24 weeks, and 86% of people on that dose got down to normal liver fat (under 5%), against 0% on placebo. The authors linked the drop to weight loss and metabolic changes. The trial had no GLP-1-only arm, so it can't say how much of that came from glucagon alone.

Whether the glucagon receptor brings side effects of its own is less clear. In TRIUMPH-1, burning or oversensitive skin (dysesthesia) showed up in 12.5% of people on 12 mg against 0.9% on placebo, while the Zepbound label puts it at 0.4% on tirzepatide 15 mg. It isn't unique to triple agonists, though: the Wegovy label lists dysesthesia in 22% of people on semaglutide 7.2 mg. In the Phase 2 trial in the New England Journal of Medicine, heart rate rose with dose, peaked at 24 weeks and eased after that. And 11.3% of people on 12 mg stopped over side effects, more than double placebo.

At the other end, retatrutide 4 mg, reached with one step up from 2 mg, averaged 19.0% weight loss at 80 weeks in TRIUMPH-1 (17.6% counting everyone), and 4.1% stopped over side effects, below placebo's 4.9%. How the trials stepped doses up is on our retatrutide dosage page.

Is GLP-3 stronger than GLP-1?

On weight loss, yes, by the numbers so far. In TRIUMPH-1, 45.3% of people on 12 mg lost 30% or more of their body weight. In STEP 1, half the people on semaglutide 2.4 mg (50.5%) lost 15% or more, a threshold half as high. In SURMOUNT-1, 57% on tirzepatide 15 mg lost 20% or more. Different trials again, and TRIUMPH-1's figure uses the stayed-on-the-drug count, which runs higher.

The top dose also brings more side effects. In TRIUMPH-1, nausea hit about 4 in 10 people (42.4%) on retatrutide 12 mg, and vomiting hit 25.3%, against 4.8% on placebo.

Our read: "stronger" fits retatrutide 12 mg, but the 4 mg result may matter more to anyone who struggled on a GLP-1, because it came within about 3 points of tirzepatide 15 mg (17.6% vs 20.9%, counted the same way, different trials) with fewer people quitting over side effects than on placebo. How those averages look week by week is on our retatrutide weight loss results page.

Questions to take to your doctor

As of September 2026, retatrutide is still in Phase 3 trials, and no approved version exists anywhere. Lilly says it plans to file a Biologics License Application with the FDA in Q1 2027, and that it's an investigational molecule that "cannot be legally sold or marketed for human use". So anything you see listed as retatrutide for sale online comes from peptide sellers, not Lilly, and Lilly warns that such products may have too much or too little active ingredient, or the wrong one entirely. For now, Lilly says it's "legally available only to participants in Lilly's clinical trials".

If you're comparing options, these are worth raising:

• I'm doing well on semaglutide or tirzepatide. What would a triple agonist add for me, given no head-to-head result is out yet? Our guides to switching from tirzepatide and switching from semaglutide cover what's known.
• I had a lot of nausea on a GLP-1. Would a lower dose or a slower increase make more sense than a stronger drug?
• Do my heart rate, liver tests or blood sugar numbers change which drug fits?
• I take insulin or a sulfonylurea. How would any of these drugs change my risk of low blood sugar?
• Is a clinical trial an option for me, and where would I find one?

Whatever you end up on, the labels for approved GLP-1 drugs give the same warning signs: belly pain that is severe and won't go away, with or without vomiting, or nausea, vomiting or diarrhea that won't stop, which can dehydrate you and strain your kidneys. Either one is a same-day call to a doctor.