Retatrutide: What the Phase 2 and Phase 3 Trial Data Actually Show

The short version

Retatrutide (also called LY3437943) is an investigational drug — meaning it is in clinical trials but not yet approved by the FDA or any other regulator. Eli Lilly is developing it for obesity, type 2 diabetes, and related metabolic conditions. What makes it different from earlier weight-loss drugs is that it activates three hormone receptors at once instead of one or two: GLP-1 (which curbs appetite), GIP (which boosts insulin release after eating), and glucagon (which raises the body's calorie burn). In a 48-week Phase 2 trial, people who received the highest dose lost an average of 24.2% of their body weight — a larger reduction than has been seen with any prior approved drug in comparable trials. Retatrutide is not available by prescription, cannot be legally dispensed outside a clinical trial, and has not been approved anywhere in the world as of mid-2026. This site summarizes the published trial data — the figures, the endpoints, and the cardiometabolic findings — with every claim cited to its source. What people report using it outside trials — and what to watch for — is on the effects page.

What the Phase 2 data measured

The core efficacy signal on retatrutide comes from two Phase 2 randomized controlled trials published in 2023, one in obesity and one in type 2 diabetes.

In the obesity trial (Jastreboff et al., N Engl J Med, 2023), 338 adults received once-weekly subcutaneous injections of retatrutide at 1, 4, 8, or 12 mg over 48 weeks. At 12 mg, mean body-weight change was −24.2% versus −2.1% with placebo ^[1]. At 8 mg, it was −17.3%. Gastrointestinal adverse events — nausea, diarrhea, vomiting — were dose-related and mostly mild to moderate. A dose-dependent heart-rate increase was documented, peaking around week 24 ^[1].

In the type 2 diabetes trial (Rosenstock et al., Lancet, 2023), 281 adults received 0.5–12 mg once weekly with stepwise dose escalation over 36 weeks. The 12 mg group showed a mean HbA1c (glycated hemoglobin — a three-month blood-glucose average) reduction of −2.02% versus −0.01% with placebo at 24 weeks, and a body-weight reduction of −16.94% versus −3.00% at 36 weeks ^[2]. No severe hypoglycemia and no deaths were recorded; mild-to-moderate GI adverse events occurred in 35% of participants ^[2].

A liver-fat substudy extended the picture further: in 98 participants with obesity and metabolic fatty liver disease (MASLD — metabolic dysfunction-associated steatotic liver disease, the current medical term for fatty liver linked to metabolic risk), retatrutide 12 mg reduced relative liver fat by −82.4% at 24 weeks, with 86% of participants reaching normal liver fat (below 5%) by MRI-PDFF (magnetic resonance imaging proton density fat fraction — a non-invasive measure of liver fat) ^[5]. The reduction was sustained to 48 weeks at −86.0% ^[5].

Triple-agonist mechanism: three receptors, one molecule

Retatrutide is a 39-amino-acid synthetic peptide engineered to activate three class-B G-protein-coupled receptors simultaneously: GLP-1R (glucagon-like peptide-1 receptor — which suppresses appetite and stimulates glucose-dependent insulin release), GIPR (glucose-dependent insulinotropic polypeptide receptor — which enhances insulin secretion after eating and influences adipose metabolism), and GCGR (glucagon receptor — which increases energy expenditure and hepatic lipid breakdown) ^[3].

Cryo-EM structures published in Cell Discovery (2024) resolved retatrutide bound to all three receptor complexes, confirming simultaneous triple engagement at 2.68 Å, 3.26 Å, and 2.84 Å resolution ^[3]. Relative to the endogenous hormones, retatrutide is approximately 8.9× more potent at GIPR, 0.4× at GLP-1R, and 0.3× at GCGR — a deliberately imbalanced profile tuned for appetite suppression plus metabolic activation ^[3].

The glucagon-receptor arm is the distinguishing feature from dual GLP-1/GIP agonists. Glucagon-receptor activation increases thermogenesis (body heat production from calorie burn), hepatic lipid metabolism, and resting energy expenditure — mechanisms that appear to underlie the larger weight-loss magnitude observed relative to dual agonists in Phase 2 ^[6]. A 2025 review in Biomolecules characterizes the ~24% weight loss at 12 mg/48 wk as a step-change versus prior incretin therapies ^[6].

Phase 3: TRIUMPH program and ongoing outcomes trials

Eli Lilly's Phase 3 program for retatrutide carries the name TRIUMPH. Multiple trials are currently enrolling or running as of mid-2026:

• TRIUMPH-3 (NCT05882045): Retatrutide in adults with obesity and established cardiovascular disease ^[9].
• NCT06383390: Dedicated cardiovascular and kidney outcomes trial — the long-term outcome evidence the class still needs ^[10].

Additional TRIUMPH arms are evaluating retatrutide in type 2 diabetes, chronic kidney disease (TRANSCEND-CKD), and in direct comparison with tirzepatide (see retatrutide vs tirzepatide for the head-to-head data context). A 2025 systematic review identified retatrutide as one of 14 agents in active Phase 3 obesity programs — the most advanced pipeline for the anti-obesity drug class ^[7].

Long-term cardiovascular, renal, and durability-of-weight-loss data from these trials are not yet available. The TRIUMPH results will determine whether the Phase 2 cardiometabolic signal translates to hard outcome benefit at scale.

What does retatrutide do

Retatrutide simultaneously activates three metabolic hormone receptors — GLP-1R, GIPR, and GCGR — producing three complementary effects: appetite suppression, glucose-dependent insulin augmentation, and increased energy expenditure. In Phase 2 trials this combination drove larger reductions in body weight, blood glucose (HbA1c), liver fat, and lipids than dual or single agonists had produced in comparable studies ^[1][2][5][6].

The key cardiometabolic data points: −24.2% body weight (12 mg, 48 wk, obesity trial) ^[1]; −2.02% HbA1c (12 mg, 24 wk, T2D trial) ^[2]; −82.4% liver fat (12 mg, 24 wk, MASLD substudy) ^[5]; improvements in lipid profiles and cardiovascular risk factors reported in an ACC 2024 analysis ^[8]. Full retatrutide results are on the results page.