How is retatrutide different from tirzepatide?

Tirzepatide is an approved dual agonist that activates the GLP-1 and GIP receptors. Retatrutide is an investigational triple agonist that activates GLP-1, GIP, and glucagon receptors. The addition of glucagon receptor activity is the structural difference. Both are developed by Eli Lilly, but they are distinct molecules with separate clinical development programs. Tirzepatide is FDA approved as Mounjaro and Zepbound. Retatrutide had not received regulatory approval as of 2025.

What is the TRIUMPH program?

TRIUMPH is Eli Lilly's phase 3 clinical trial program for retatrutide. It comprises multiple trials across the compound's intended indications, including obesity and type 2 diabetes. Phase 3 trials are larger and longer than the phase 2 studies and are designed to generate the data required for a regulatory submission to the FDA and other agencies.

What did the NEJM phase 2 trial show?

The phase 2 obesity trial published in the New England Journal of Medicine in June 2023 reported that participants in the highest dose group experienced an average body weight reduction of approximately 24 percent at 48 weeks, compared with approximately 2 percent with placebo. The trial was authored by Jastreboff et al. and was a randomized, placebo-controlled dose-finding study. Phase 2 data is not the same as phase 3 data, and comparisons across different studies and compounds carry important limitations.

Why does adding glucagon receptor activity help with weight loss?

Native glucagon raises blood glucose, which seems counterintuitive in a metabolic therapy. However, glucagon also promotes fat breakdown and appears to increase energy expenditure through effects on liver metabolism and brown adipose tissue. When GLP-1 and GIP receptor activity is simultaneously present to moderate the glucose-raising effects, researchers hypothesize that controlled glucagon receptor stimulation adds a fat-burning signal through a pathway distinct from the incretin effects. Phase 3 data will test whether this produces clinically meaningful additional benefit.

Is retatrutide approved for any use?

No. As of 2025, retatrutide had not been approved by the FDA or any other regulatory agency for any indication. It remains investigational, with phase 3 trials ongoing under the TRIUMPH program. Material sold as research retatrutide is a research chemical intended for laboratory use only and is not cleared for human administration.

Is retatrutide the same as the 'triple agonist' mentioned in the press?

Yes. Retatrutide is the compound commonly referred to in medical and general press coverage as a triple agonist or triple incretin receptor agonist. It is also known by its Lilly designation LY3437943. Other triple agonist compounds are in earlier stages of research at other companies, but retatrutide is the one with published phase 2 data from the NEJM 2023 paper and an active phase 3 program.

Retatrutide and the TRIUMPH Trials: Triple Agonist Research

Published: May 8, 2026•Updated: May 8, 2026

Research use only. This article discusses compounds that include approved medications, investigational drugs, and research peptides. Material sold for research is not cleared for human administration and is not a substitute for medical advice.

Retatrutide is an investigational peptide developed by Eli Lilly that activates three hormone receptors at once: GLP-1, GIP, and glucagon. Adding glucagon receptor activity to the incretin combination is a deliberate pharmacological move, not an oversight, and it changes the character of how the drug works compared with the dual agonists that came before it. Phase 2 data published in the New England Journal of Medicine in 2023 drew wide attention by reporting average body weight reductions of approximately 24 percent at 48 weeks at the highest dose tested. This article explains why researchers added glucagon to the mix, what the trial data shows so far, and where the TRIUMPH phase 3 program stands.

What retatrutide is

Retatrutide, also designated LY3437943 in Lilly's internal nomenclature, is a single synthetic peptide that was engineered to bind and activate the GLP-1 receptor, the GIP receptor, and the glucagon receptor simultaneously. Existing approved therapies target one or two of those receptors. Semaglutide activates only the GLP-1 receptor. Tirzepatide activates GLP-1 and GIP receptors together. Retatrutide adds glucagon receptor agonism on top of both, making it the first triple agonist of this class to reach clinical development.

The molecule was designed as a once weekly subcutaneous injection, matching the dosing schedule of semaglutide and tirzepatide. It is a modified peptide with structural features that slow enzymatic breakdown and extend circulating half-life, similar to the engineering strategy used for the longer acting GLP-1 analogs that preceded it. Retatrutide has no approved indication as of 2025 and is classified as an investigational drug.

Retatrutide is not tirzepatide

Tirzepatide (Mounjaro and Zepbound) is Eli Lilly's approved dual GIP and GLP-1 agonist. Retatrutide is a distinct investigational compound from the same company that adds glucagon receptor activity. They have different amino acid sequences, different receptor profiles, and different clinical development programs. The fact that both come from Lilly and both involve the GIP and GLP-1 receptors creates confusion in popular coverage, but they are separate molecules.

Why glucagon was added

Glucagon is typically framed as the counterpart to insulin: insulin lowers blood glucose, glucagon raises it by promoting glucose release from the liver. Adding a glucagon receptor agonist to a weight loss compound might seem counterproductive at first, because uncontrolled glucagon signaling in type 2 diabetes contributes to elevated fasting glucose.

The rationale for including glucagon activity at specific doses in a GIP and GLP-1 combination rests on glucagon's role in fat metabolism and energy expenditure. When GLP-1 and GIP receptor activity is already present to control insulin secretion and moderate the glucose-raising effects of glucagon, researchers hypothesized that co-activating the glucagon receptor would add a meaningful signal through a separate pathway: increased lipolysis in fat tissue and a lift in energy expenditure through effects on the liver and brown adipose tissue.

GLP-1 receptor activity

Stimulates glucose-dependent insulin secretion, suppresses glucagon from alpha cells, slows gastric emptying, and signals satiety through central nervous system receptors. The foundational mechanism shared with semaglutide.

GIP receptor activity

Augments insulin secretion in a glucose-dependent manner and appears to act synergistically with GLP-1 in central appetite circuits. The same pathway targeted by the GIP component of tirzepatide.

Glucagon receptor activity

In the presence of GLP-1 and GIP activity, controlled glucagon receptor stimulation is hypothesized to increase energy expenditure through liver and brown adipose tissue effects and promote fat utilization beyond what the incretin pathways alone produce.

Whether the glucagon component produces meaningful additive effects beyond what tirzepatide achieves is the central scientific question the TRIUMPH trials are designed to test. Phase 2 results suggested the answer was yes, but the rigorous comparison requires phase 3.

Phase 2 NEJM findings

The phase 2 trial of retatrutide in obesity was published in the New England Journal of Medicine in June 2023. The study was a multicenter, randomized, placebo-controlled dose-finding trial that enrolled adults with a body mass index of 30 or higher, or 27 or higher with at least one weight-related condition, over 48 weeks.

Participants were randomized to one of several dose levels of retatrutide or placebo. At the highest dose tested, the trial reported an average body weight reduction of approximately 24 percent from baseline at 48 weeks, compared with approximately 2 percent in the placebo group. The results across dose levels showed a consistent dose-response relationship, with higher doses producing larger weight reductions.

Context for the 24 percent figure

The approximately 24 percent average weight reduction at 48 weeks in the highest dose group drew significant attention because it exceeded what had previously been reported at comparable time points for semaglutide or tirzepatide in phase 2 data. Semaglutide phase 3 STEP 1 reported approximately 14.9 percent average weight loss at 68 weeks. Tirzepatide SURMOUNT-1 reported up to approximately 22.5 percent at 72 weeks. Direct comparisons between phase 2 data for one drug and phase 3 data for another carry important limitations, including different study populations, durations, and protocols.

A parallel phase 2 trial in adults with type 2 diabetes was also published in 2023, reporting reductions in HbA1c alongside body weight effects. These results supported the view that retatrutide, like the incretin therapies before it, produced meaningful effects on both glycemic control and body composition in populations where both concerns are present.

•Phase 2 obesity trial published in NEJM June 2023 (Jastreboff et al.)
•48 week duration; placebo-controlled; multiple dose levels tested
•Highest dose group reported approximately 24 percent average body weight reduction at 48 weeks
•Dose-response relationship observed across tested dose levels
•Parallel type 2 diabetes trial reported HbA1c reductions alongside weight effects
•Results supported advancement to phase 3 development

The TRIUMPH phase 3 program

Eli Lilly launched the TRIUMPH trial program to evaluate retatrutide in phase 3 clinical development. The program comprises multiple trials examining retatrutide across its intended patient populations, including people with obesity without type 2 diabetes, people with both conditions, and people with cardiovascular risk.

Phase 3 trials are substantially larger than the phase 2 studies and are designed with the statistical power needed to support regulatory submissions. They test active doses against placebo and in some cases against active comparators. The TRIUMPH program is expected to generate the safety and efficacy data that would be required for FDA and international regulatory review.

As of mid-2025, the TRIUMPH trials were ongoing. Topline results from key trials had not been publicly reported, and no regulatory submission had been filed. The timeline for potential approval depends on trial completion and the regulatory review process, neither of which had concluded. Retatrutide remains investigational.

Safety signals from phase 2

The adverse event profile observed in retatrutide phase 2 trials resembled what has been seen with other incretin-based therapies. Nausea, vomiting, and diarrhea were among the most frequently reported adverse events, consistent with the gastrointestinal effects that accompany GLP-1 receptor activation and slowed gastric emptying.

The dose-dependent weight loss observed in retatrutide trials was accompanied by a consistent pattern: higher doses produced larger effects but also higher rates of gastrointestinal adverse events. This is a familiar tradeoff in GLP-1 pharmacology, where dose escalation protocols are designed specifically to reduce early tolerability issues by giving the body time to adjust.

•Nausea, vomiting, and diarrhea were the most commonly reported adverse events in phase 2
•Adverse event rates were dose-dependent, with higher rates at higher doses
•Injection site reactions were reported in a subset of participants
•Gallbladder-related events have been observed with incretin therapies generally and were monitored in retatrutide trials
•Phase 2 trials are not powered for rare events; phase 3 data will provide a more complete safety picture
•No approved prescribing information exists; material sold as research retatrutide carries additional uncertainty about identity and purity

Regulatory and competitive context

Retatrutide has no approved indication from the FDA, the European Medicines Agency, or any comparable regulatory authority as of 2025. It is not available as a prescription medication. Material sold under the retatrutide name is classified as a research chemical intended strictly for laboratory use. It is not cleared for human administration, and there is no established safety profile from controlled human trials beyond the phase 2 setting.

The competitive context matters for understanding where retatrutide fits. Tirzepatide, Lilly's approved dual agonist, already holds FDA approval for type 2 diabetes (Mounjaro) and obesity (Zepbound) and is performing well commercially. Novo Nordisk's semaglutide dominates the GLP-1 space with Ozempic and Wegovy. A triple agonist that produces larger average weight loss would address a genuine unmet need: the subset of patients who do not achieve adequate response on existing approved therapies.

From an anti-doping perspective, the World Anti-Doping Agency prohibits GLP-1 receptor agonists and related peptide hormones under its Prohibited List. This prohibition would be expected to cover retatrutide for athletes subject to WADA rules, consistent with how other incretin therapies have been treated.