Five trials and three Phase 3 programs: the complete published retatrutide research record.

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Retatrutide is an investigational drug being tested for obesity and type 2 diabetes. To understand the research, one core fact matters: it works on three hormone systems at once, not one. Most drugs for obesity or diabetes work on a single pathway. Retatrutide was engineered to activate GIP (a hormone that helps control insulin after meals), GLP-1 (a hormone that reduces appetite), and glucagon (a hormone that raises calorie burning) — all at the same time, through a single weekly injection.

Researchers have now completed four significant trials: a Phase 1b safety study, two Phase 2 efficacy studies (obesity; type 2 diabetes), and a Phase 2 liver-fat study. The headline finding from the Phase 2 obesity trial — a mean 24.2% body-weight reduction at 48 weeks in people with obesity — is the largest weight loss figure reported by any medicine in a Phase 2 trial of that length ^[1]. This page documents those trials in detail. Retatrutide references lists every cited source.

Mechanism: how the triple-agonist pharmacology was established

The pharmacological case for a triple agonist was built on a straightforward observation: GLP-1 single agonism reduces appetite; adding GIP agonism (dual incretin action) adds further efficacy; adding glucagon receptor agonism on top contributes thermogenic energy expenditure that neither incretin arm produces alone ^[6].

Retatrutide is a 39-amino-acid synthetic peptide engineered on a GIP-based backbone, with a C20 fatty-diacid acyl chain added to bind albumin and extend its half-life ^[4]. The structural pharmacology was confirmed in a 2024 cryo-EM study that resolved retatrutide simultaneously bound to all three class-B GPCRs (G-protein-coupled receptors — a broad family of cell-surface signal proteins). The cryo-EM structures showed retatrutide's relative potencies: approximately 8.9 times more potent than native GIP at the GIPR, about 40% of native GLP-1 at GLP-1R, and about 30% of native glucagon at GCGR ^[3]. The deliberate asymmetry — maximal GIP engagement, attenuated glucagon tone — is what allows thermogenic benefit without unacceptable glycemic volatility.

Downstream, all three receptor activations converge on cAMP/PKA signaling — a shared second-messenger cascade that translates the hormone signal into cellular action. The triple-simultaneous activation via a single molecule is what distinguishes retatrutide from prior agents.

Phase 1b: first-in-human pharmacokinetics and early efficacy

The first-in-human Phase 1b trial enrolled 72 adults with type 2 diabetes (HbA1c 7.0–10.5%) across dose cohorts of 0.5, 1.5, 3, 3/6, and 3/6/9/12 mg subcutaneously once weekly for 12 weeks ^[4].

The pharmacokinetic (PK) data established a half-life of approximately 6 days, confirming that once-weekly subcutaneous dosing would maintain therapeutic concentrations between injections — the foundational PK rationale for the weekly injection schedule ^[4]. The retatrutide half life page covers the PK data in detail.

Efficacy signals emerged even at the short Phase 1b duration: at the highest dose cohort, participants lost a placebo-adjusted mean of -8.96 kg (90% CI: −11.16 to −6.75) over 12 weeks ^[4]. Daily glucose fell by 2.8 mmol/L at the 3 mg dose. Treatment-emergent adverse events occurred in 63% of participants, predominantly gastrointestinal. No severe adverse events were reported. The safety profile was characterized as acceptable and sufficient to support Phase 2 development.

Phase 2 obesity trial: the 24.2% finding

The pivotal Phase 2 obesity trial (Jastreboff AM et al., N Engl J Med 2023) enrolled 338 adults with obesity (BMI ≥30, or 27–<30 with a weight-related comorbidity) and randomized them to once-weekly subcutaneous doses of 1, 4, 8, or 12 mg or placebo for 48 weeks ^[1].

The primary endpoint — mean body-weight change at 48 weeks — showed a dose-response relationship: −8.7% at 1 mg, −17.3% at 4 mg, −22.8% at 8 mg, and −24.2% at 12 mg, versus −2.1% for placebo ^[1]. A pooled systematic review of retatrutide clinical trials (Misra et al. 2025) confirmed the 12 mg dose as consistently maximally effective for weight, BMI, and waist circumference across pooled trial data ^[7].

Gastrointestinal adverse events were dose-related: nausea, constipation, diarrhea, and vomiting were all most common at the 12 mg dose. Discontinuation due to adverse events was 18% at the highest dose. A dose-dependent heart-rate increase peaking at approximately 24 weeks was also observed ^[1].

Retatrutide results in the Phase 2 obesity trial are summarized further on the retatrutide results page.

Phase 2 type 2 diabetes trial

The Phase 2 T2D trial (Rosenstock J et al., Lancet 2023) enrolled 281 adults with type 2 diabetes and randomized them to once-weekly subcutaneous doses from 0.5–12 mg (with stepwise dose escalation) or placebo for 36 weeks ^[2].

At the 12 mg dose, retatrutide reduced HbA1c (glycated hemoglobin — a 3-month blood glucose average) by −2.02% versus −0.01% for placebo at 24 weeks, and reduced body weight by −16.94% versus −3.00% for placebo at 36 weeks ^[2]. Mild-to-moderate GI adverse events occurred in 35% of participants. No severe hypoglycemia was reported; no deaths occurred.

The results established that retatrutide's triple-agonist mechanism produced simultaneous glycemic and weight benefits in a T2D population without the safety signals that would have halted development. These findings supported the TRIUMPH Phase 3 program.

Phase 2 MASLD substudy: liver fat findings

A Phase 2a substudy (Sanyal AJ et al., Nature Medicine 2024) enrolled 98 participants with obesity or overweight and MASLD (metabolic dysfunction-associated steatotic liver disease — formerly called non-alcoholic fatty liver disease, or NAFLD), with ≥10% liver fat by MRI-PDFF (a non-invasive imaging measure of liver fat content) and no type 2 diabetes ^[5].

At 24 weeks, the 12 mg dose group showed a relative liver-fat reduction of −82.4% from baseline; 86% of participants in that arm reached a normal liver fat threshold (<5%) ^[5]. Reductions were sustained to 48 weeks (−86.0% at 12 mg). Efficacy was dose-graded: −42.9% at 1 mg, −57.0% at 4 mg, −81.4% at 8 mg. Placebo change was +0.3%. These are among the largest liver-fat reductions reported in any pharmacological trial.

The findings are significant because MASLD is a common metabolic comorbidity in obesity, and no pharmacological treatment has previously shown reductions of this magnitude. Dedicated MASLD outcome trials are part of the broader retatrutide development program.

Phase 3: the TRIUMPH program

Eli Lilly's TRIUMPH program is the Phase 3 development package for retatrutide. As of mid-2026, three pivotal efficacy trials are ongoing:

• TRIUMPH-1 (NCT05929066): adults with obesity without diabetes, the core pivotal obesity trial ^[9]
• TRIUMPH-2 (NCT05931367): adults with obesity and type 2 diabetes ^[10]
• TRIUMPH-3 (NCT05882045): adults with obesity and established cardiovascular disease ^[11]

The first Phase 3 result to reach publication is from a separate program: TRANSCEND-T2D-1 (Bajaj HS et al., Lancet 2026) enrolled 537 adults with type 2 diabetes and inadequate glycemic control on diet and exercise alone ^[12]. Retatrutide 12 mg reduced HbA1c by a mean of 1.94% and body weight by 15.3% from baseline versus −0.81% HbA1c and −2.6% body weight for placebo at 40 weeks. Adverse events were predominantly mild-to-moderate GI. No severe hypoglycemia was reported. Discontinuation due to adverse events occurred in 9.8% of retatrutide participants versus 2.2% placebo.

Long-term cardiovascular outcomes and renal safety trials are also registered and ongoing.

Retatrutide vs tirzepatide: what the comparative data shows

No head-to-head trial comparing retatrutide and tirzepatide has been published as of mid-2026. A Phase 3 active-comparator trial is registered, but has not reported. All comparisons in the published literature are descriptive — parallel trial data, not randomized head-to-head.

A 2025 meta-analysis (Chan ZH et al., Cardiology in Review 2026) pooled 10 RCTs of incretin polyagonists (including tirzepatide, retatrutide, and mazdutide) versus placebo and found a pooled mean weight reduction of −11.47 kg (95% CI: −14.00 to −8.95) for the class as a whole, with significant reductions in waist circumference, HbA1c, and fasting glucose ^[13].

A 2025 systematic review of GLP-1-class agents for weight loss without diabetes (Moiz A et al., Annals of Internal Medicine 2025) described retatrutide 12 mg as reporting the largest single weight-loss figure (22.1%) among agents evaluated, with the note that this was a descriptive comparison, not a formal head-to-head ranking, and no meta-analysis was performed across agents ^[14].

The triagonist mechanism — the addition of glucagon receptor agonism to the dual incretin base — is the proposed explanation for the larger weight reduction signal. The glucagon arm adds thermogenic energy expenditure that neither GLP-1 nor GIP agonism produces alone.