# Retatrutide Research: Phase 1 through Phase 3 Trial Data > Retatrutide research from Phase 1b through the TRANSCEND-T2D-1 Phase 3 trial. Mechanism, efficacy, liver-fat, body composition, and the TRIUMPH program — every claim cited. Phase 1b through Phase 3. Obesity, type 2 diabetes, liver disease, and the body-composition record. Primary citations throughout. ## The short version Retatrutide is an investigational obesity and diabetes drug that is not approved anywhere as of 2026. It works by activating three hormone receptors at once — GIP, GLP-1, and glucagon — using a single injected molecule. Phase 2 clinical trials have produced the largest weight-loss numbers ever reported in a randomized drug trial: up to 24.2% of body weight at 48 weeks. A Phase 3 program (TRIUMPH) is now underway, and the first Phase 3 result was published in 2026. This page summarizes the full published [Retatrutide research](/research) record in order: how the molecule was engineered and how it works, what Phase 1b showed about its pharmacokinetics, what Phase 2 found in obesity and diabetes, what the liver-disease substudy found, and where the Phase 3 program stands. All numbers are cited to the study that produced them. ## Mechanism: triple-receptor agonism Retatrutide (LY3437943) is a 39-amino-acid synthetic peptide engineered to activate three class-B GPCRs (G protein-coupled receptors — cell-surface proteins that relay hormone signals into the cell) simultaneously: the GLP-1 receptor (GLP-1R), the GIP receptor (GIPR), and the glucagon receptor (GCGR). Cryo-EM structures resolved the binding geometry at all three receptors. Retatrutide is approximately 8.9 times more potent at GIPR than native GIP, and 0.3× to 0.4× at GCGR and GLP-1R compared to their endogenous hormones [3]. The design amplifies the GIPR arm (which drives insulinotropic and adipose effects) while tempering the glucagon arm (to limit hyperglycemia) — a deliberate pharmacodynamic trade-off. The GLP-1R component suppresses appetite and slows gastric emptying. The GIPR component augments glucose-dependent insulin secretion and influences adipose metabolism. The GCGR component increases energy expenditure through hepatic lipid metabolism and thermogenesis. Together, the three arms produce a caloric deficit larger than any single-agonist approach has achieved in comparable studies [8, 11]. A 2024 review characterizes the tri-agonist mechanism as the pharmacological basis for weight reductions rivaling bariatric surgery [11]. ## Phase 1b: pharmacokinetics and first efficacy signal The first-in-human Phase 1b trial enrolled 72 adults with type 2 diabetes and measured pharmacokinetics (how the drug moves through the body), tolerability, and early efficacy at doses from 0.5 mg to 12 mg once weekly over 12 weeks [4]. Key findings: retatrutide has a half-life of approximately 6 days, supporting once-weekly dosing. The highest-dose group lost a placebo-adjusted 8.96 kg (90% CI: −11.16 to −6.75 kg) over 12 weeks. Daily glucose fell by 2.8 mmol/L at 3 mg. Treatment-emergent adverse events (TEAEs) occurred in 63% of participants, mostly GI in nature. No severe hypoglycemia was recorded [4]. Phase 1b established the once-weekly dosing schedule and the dose escalation approach used in all subsequent trials. ## Phase 2: obesity (48-week trial) The Phase 2 obesity trial enrolled 338 adults with a BMI of 30 or above (or 27–30 with a weight-related comorbidity), without diabetes. Participants received 1, 4, 8, or 12 mg once weekly via subcutaneous injection for 48 weeks, with stepwise dose escalation [1]. Results at 48 weeks: - Mean body-weight change: −24.2% at 12 mg versus −2.1% for placebo [1]. - Waist circumference: reduced by approximately 19.6 cm at 12 mg. - GI adverse events: dose-related, mostly mild-to-moderate; nausea affected up to 45% at 12 mg. - Heart rate: dose-dependent increase peaking around week 24, then partially stabilizing. - Discontinuation due to AEs: 18% at the 12 mg dose. A 2025 commentary on this trial characterizes the up to 24% weight loss as a step-change in the incretin-pharmacotherapy class [6]. A 2024 review notes that tri-agonist agents, of which retatrutide is the lead example, approach the weight-loss magnitude historically associated only with bariatric surgery [11]. ## Phase 2: type 2 diabetes (36-week trial) The Phase 2 type 2 diabetes trial enrolled 281 adults inadequately controlled on background antidiabetic medication. Doses ranged from 0.5 to 12 mg once weekly with stepwise escalation over 36 weeks [2]. Results: - HbA1c reduction at 24 weeks: −2.02% at 12 mg versus −0.01% for placebo [2]. - Body weight at 36 weeks: −16.94% at 12 mg versus −3.00% for placebo. - Mild-to-moderate GI AEs in 35% of participants. - No severe hypoglycemia, no deaths [2]. The glycemic and weight results in the diabetic population are consistent with — and at 12 mg largely comparable to — the obesity cohort, establishing that the weight-loss effect persists in individuals with insulin resistance. ## Phase 2a: liver disease (MASLD substudy) A dedicated Phase 2a substudy enrolled 98 participants with obesity or overweight and MASLD (metabolic dysfunction-associated steatotic liver disease — the current term for fatty liver linked to metabolic risk factors, formerly called NAFLD), with confirmed liver fat of at least 10% by MRI-PDFF (magnetic resonance imaging proton density fat fraction, a non-invasive measure of liver-fat content), without type 2 diabetes [5]. Results at 24 weeks: - Relative liver-fat change at 12 mg: −82.4% versus +0.3% for placebo [5]. - Proportion reaching normal liver fat (<5%): 86% at 12 mg. - Reductions sustained and deepened to 48 weeks (−86.0% at 12 mg). These are the largest liver-fat reductions reported in an incretin-class trial. An incretin-based therapy review situates retatrutide among agents showing broad cardiometabolic benefit across MASLD and related conditions [9]. ## Phase 3: TRANSCEND-T2D-1 (first published Phase 3 result) TRANSCEND-T2D-1 is the first Phase 3 trial to report results. It enrolled 537 adults with type 2 diabetes inadequately controlled with diet and exercise over 40 weeks [12]. Primary results: - HbA1c reduction: −1.94% at 12 mg versus −0.81% for placebo [12]. - Body weight: −15.3% at 12 mg versus −2.6% for placebo. - Adverse events: predominantly mild-to-moderate GI; treatment discontinuation due to AEs in 8.3% of the retatrutide arm. - No severe hypoglycemia reported. These Phase 3 findings are directionally consistent with Phase 2 results, providing early confirmation that the efficacy signal replicates in a larger pre-approval trial. Additional TRIUMPH trials — obesity, cardiovascular outcomes, CKD outcomes, and an active comparator versus tirzepatide — remain ongoing [8]. ## Retatrutide vs tirzepatide A head-to-head trial of retatrutide versus tirzepatide is part of the TRIUMPH program and has not reported results as of mid-2026 [8]. Any comparison made before those data exist is indirect and depends on cross-trial assumptions that may not hold. What can be said from the published record: the Phase 2 obesity trial for retatrutide (−24.2% at 12 mg/48 wk) produced numerically larger reductions than tirzepatide's SURMOUNT-1 Phase 3 results in a comparable population. A 2025 review characterizes retatrutide as achieving weight reduction exceeding tirzepatide based on currently available data, while noting the direct comparison is pending [8]. The 2026 multi-receptor review identifies weight regain upon discontinuation as a class-level limitation shared by current GLP-1-based therapies, underscoring that no current agent — tirzepatide or retatrutide — has demonstrated persistent weight loss after stopping [13]. Generic names used throughout: tirzepatide is the INN. No brand names appear on this site. --- A risograph broadside reading of the retatrutide Phase 2 and Phase 3 trial record — discontinuation, durability, and the stopping question set down precisely and cited to source, with no clinic and no prescription behind the charcoal stock.