Semaglutide, tirzepatide and retatrutide are three generations of one idea. Semaglutide activates a single receptor, GLP-1R. Tirzepatide adds the GIP receptor. Retatrutide adds the glucagon receptor on top of both. Across the pivotal trials, mean weight reduction rose with each added receptor — roughly 15% at 68 weeks for semaglutide, 21% at 72 weeks for tirzepatide, and 24% at 48 weeks for retatrutide — with the important caveat that those are three different trials in different populations. Only tirzepatide and semaglutide have been compared head-to-head, and tirzepatide won on both glycemic and weight endpoints. Retatrutide remains investigational. The details are below; for the biology of the incretin system itself see the GLP-1 and incretin peptides profile.
Three receptors, three generations
GLP-1R — glucose-dependent insulin secretion, glucagon suppression, slowed gastric emptying and central satiety signaling. This is the receptor the whole class was built on, and it accounts for most of the appetite effect.
GIPR — the receptor for the other incretin, glucose-dependent insulinotropic polypeptide. Its contribution is the most debated in the field: GIP receptor agonism adds insulinotropic and adipose-tissue effects, appears to improve GI tolerability of GLP-1 agonism in some models, and — confusingly — GIPR antagonism also reduces body weight in mice. Tirzepatide is deliberately “imbalanced” toward GIPR: its affinity there is comparable to native GIP, while its GLP-1R potency is several-fold below native GLP-1.
GCGR — the glucagon receptor. Glucagon raises energy expenditure and hepatic fat oxidation but also raises blood glucose; the design premise of a triple agonist is that concurrent GLP-1/GIP activity offsets the hyperglycemic side while the energy-expenditure side is kept. Retatrutide is engineered to be most potent at GIPR, then GLP-1R, with glucagon-receptor activity the weakest of the three.
Structure and pharmacokinetics
| Semaglutide | Tirzepatide | Retatrutide | |
|---|---|---|---|
| Receptors | GLP-1R | GIPR + GLP-1R | GIPR + GLP-1R + GCGR |
| Backbone | GLP-1(7–37) analog, 31 residues | GIP-based, 39 residues | GIP-based, 39 residues |
| Key modifications | Aib8 (DPP-4 resistance), Arg34, C18 fatty-diacid on Lys26 via a linker (albumin binding) | Aib2 and Aib13, C20 fatty-diacid on Lys20 via a linker | Aib substitutions, C20 fatty-diacid via a linker |
| Half-life | ≈ 1 week | ≈ 5 days | ≈ 6 days |
| Trial dosing interval | Once weekly | Once weekly | Once weekly |
| Developer / first described | Novo Nordisk, 2015 (Lau et al.) | Eli Lilly, 2018 (Coskun et al., “LY3298176”) | Eli Lilly, 2022 (Coskun et al., “LY3437943”) |
All three share the same engineering: a DPP-4-resistant N-terminus and a fatty-acid “tail” that binds albumin, which is what turns a peptide with a half-life of minutes into a once-weekly molecule. The tail is also why these peptides are prone to aggregation in solution — relevant to anyone handling them in the lab (see How to store peptides).
What the trials showed
| Trial | Population | Duration | Result |
|---|---|---|---|
| STEP 1 (semaglutide 2.4 mg) | 1,961 adults with overweight/obesity, no diabetes | 68 weeks | −14.9% body weight vs −2.4% placebo |
| SURMOUNT-1 (tirzepatide) | 2,539 adults with obesity, no diabetes | 72 weeks | −15.0% / −19.5% / −20.9% at 5 / 10 / 15 mg vs −3.1% placebo |
| Retatrutide Phase 2 | 338 adults with obesity, no diabetes | 48 weeks | −24.2% at 12 mg vs −2.1% placebo (−17.5% already at 24 weeks) |
| SURPASS-2 (head-to-head, T2D) | 1,879 adults with type 2 diabetes on metformin | 40 weeks | Tirzepatide 15 mg: HbA1c −2.30%, weight −11.2 kg; semaglutide 1 mg: −1.86%, −5.7 kg |
| SURMOUNT-5 (head-to-head, obesity) | 751 adults with obesity, no diabetes | 72 weeks | Tirzepatide −20.2% vs semaglutide −13.7% |
Two cautions when reading that table. First, cross-trial comparison is not head-to-head comparison: populations, durations and titration schedules differ, and retatrutide’s 48-week number comes from a Phase 2 study of a few hundred participants rather than a Phase 3 of thousands. Second, semaglutide has the deepest outcomes evidence — the SELECT trial showed a 20% relative reduction in major adverse cardiovascular events in people with obesity and established cardiovascular disease but no diabetes — and that kind of hard-endpoint data does not yet exist for retatrutide.
Where each stands clinically
- Semaglutide — approved for type 2 diabetes (2017) and chronic weight management (2021), with a cardiovascular-risk-reduction indication following SELECT. The most-studied molecule in the class.
- Tirzepatide — approved for type 2 diabetes (2022) and chronic weight management (2023), with obstructive sleep apnea added in 2024.
- Retatrutide — investigational. The Phase 3 program (TRIUMPH) in obesity, type 2 diabetes and related conditions is ongoing; no regulatory approval anywhere as of this writing.
The research-relevant differences
- Receptor pharmacology is the whole story. An assay that reads out GLP-1R activation alone cannot distinguish the three; a comparison needs GIPR and GCGR readouts as well, and the imbalanced potencies mean equimolar comparisons are not equipotent ones.
- Energy expenditure. The glucagon component is retatrutide’s distinguishing hypothesis. In preclinical models the triple agonist raised energy expenditure where the dual agonist mainly reduced intake; whether that translates into a different body-composition outcome in humans is an open Phase 3 question.
- Lean mass. A live question for the whole class — how much of the weight lost is fat-free mass, and whether the answer differs between molecules. No trial has been designed to settle it.
- Tolerability. Gastrointestinal effects are class-wide and dose-titration-dependent; the head-to-head trials did not show a clear tolerability winner.
- Handling. As lipidated, amphipathic peptides all three are aggregation-prone; the same lab practices apply to each.
Reading the product names
Our catalog uses coded names — GLP-1-S, GLP-2-T, GLP-3-R — where the number is a catalog code for the generation and the letter is the compound: Semaglutide (10 mg, 20 mg), Tirzepatide (15 mg, 20 mg, 30 mg) and Retatrutide (10 mg, 20 mg, 30 mg). Note in particular that GLP-2-T is tirzepatide, not GLP-2 — GLP-2 is a separate intestinal hormone with its own analog (teduglutide) and no role in this comparison. Each product carries its lot’s certificate of analysis; this guide explains how to read one.
Frequently asked questions
Straight answers to the questions researchers most often ask. Everything below summarizes published preclinical and clinical literature; it is not medical advice, and these compounds are supplied for laboratory research only.
Is retatrutide stronger than tirzepatide?
In its Phase 2 trial retatrutide produced a larger mean weight reduction (−24.2% at 48 weeks) than tirzepatide did in SURMOUNT-1 (−20.9% at 72 weeks), but those are different trials in different populations, and retatrutide's number comes from a few hundred participants rather than a completed Phase 3. There is no head-to-head trial of the two, so "stronger" is a reasonable hypothesis rather than an established fact.
Is tirzepatide better than semaglutide?
In the two head-to-head trials, yes on the measured endpoints: SURPASS-2 (type 2 diabetes) showed greater HbA1c and weight reduction with tirzepatide 15 mg than semaglutide 1 mg, and SURMOUNT-5 (obesity) showed −20.2% versus −13.7% body weight at 72 weeks. Semaglutide has the larger body of cardiovascular-outcome evidence.
What is the difference between GLP-1 and GIP?
Both are incretin hormones released from the gut after a meal that enhance glucose-dependent insulin secretion. GLP-1 also suppresses glucagon, slows gastric emptying and reduces appetite centrally; GIP acts on pancreatic beta cells and adipose tissue and its role in body weight is still debated. Semaglutide targets GLP-1R only; tirzepatide and retatrutide target both.
Is retatrutide FDA approved?
No. Retatrutide is an investigational compound in Phase 3 trials (the TRIUMPH program). Semaglutide and tirzepatide are approved medicines; retatrutide is supplied strictly as a research compound.
What does "triple agonist" mean?
A single molecule that activates three receptors — for retatrutide, the GIP, GLP-1 and glucagon receptors. The design aims to combine the appetite and glycemic effects of incretin agonism with the energy-expenditure effects of glucagon-receptor activation.
Why is GLP-2-T tirzepatide and not GLP-2?
The catalog names are codes: the number is the catalog generation and the letter is the compound (S for semaglutide, T for tirzepatide, R for retatrutide). GLP-2 itself is a different intestinal hormone and is not part of this series.
Do all three have a one-week half-life?
Approximately: about a week for semaglutide, about five days for tirzepatide and about six days for retatrutide. All three were dosed once weekly in their trials, which is the property the fatty-acid "tail" on each molecule was engineered for.
Can research on one of these peptides be applied to the others?
Only partly. They share the GLP-1 receptor mechanism and the same pharmacokinetic engineering, so handling, stability and GLP-1R-dependent effects generalize reasonably. Anything involving GIP or glucagon receptor signaling does not, and the three differ in potency at each receptor.
References
- Once-Weekly Semaglutide in Adults with Overweight or Obesity STEP 1: −14.9% vs −2.4% at 68 weeks.
- Tirzepatide Once Weekly for the Treatment of Obesity SURMOUNT-1: −20.9% at 15 mg vs −3.1% at 72 weeks.
- Triple-Hormone-Receptor Agonist Retatrutide for Obesity - A Phase 2 Trial −24.2% at 12 mg vs −2.1% at 48 weeks.
- Tirzepatide versus Semaglutide Once Weekly in Patients with Type 2 Diabetes SURPASS-2 head-to-head.
- Tirzepatide as Compared with Semaglutide for the Treatment of Obesity SURMOUNT-5 head-to-head: −20.2% vs −13.7%.
- Semaglutide and Cardiovascular Outcomes in Obesity without Diabetes SELECT: 20% relative MACE reduction.
- Discovery of the Once-Weekly Glucagon-Like Peptide-1 (GLP-1) Analogue Semaglutide Structure and design rationale.
- LY3298176, a novel dual GIP and GLP-1 receptor agonist for the treatment of type 2 diabetes mellitus: From discovery to clinical proof of concept Tirzepatide discovery; GIP-biased receptor pharmacology.
- LY3437943, a novel triple glucagon, GIP, and GLP-1 receptor agonist for glycemic control and weight loss: From discovery to clinical proof of concept Retatrutide discovery; receptor potency ordering and Phase 1 pharmacokinetics.
- Mechanisms of action and therapeutic applications of GLP-1 and dual GIP/GLP-1 receptor agonists
- Retatrutide in type 2 diabetes mellitus and obesity: an overview Current status of the Phase 3 program.
