TRIUMPH-1 Phase 3: Retatrutide's Pivotal Obesity Trial Results (2026)

What is the TRIUMPH-1 trial?

TRIUMPH-1 is Eli Lilly’s flagship Phase 3 randomised, double-blind, placebo-controlled trial of retatrutide (LY3437943) in adults with obesity or overweight. It is the largest and longest registrational study in the TRIUMPH programme and the trial against which regulatory submissions will primarily be assessed. Topline results were announced on 21 May 2026; the full dataset was presented at the 86th American Diabetes Association (ADA) Scientific Sessions in New Orleans on 6 June 2026.[1]

The acronym TRIUMPH stands for TRIple-hormone-receptor aGonist for ObesIty Management Pivotal. The programme spans multiple trials across different metabolic phenotypes; TRIUMPH-1 specifically enrolled participants without type 2 diabetes, isolating the pharmacological response of retatrutide’s triple-receptor mechanism in a non-diabetic obesity model. For the parallel T2D dataset, see the companion guide: Retatrutide in Type 2 Diabetes: TRANSCEND-T2D-1.

Retatrutide (CAS 2381089-83-2, also known as LY3437943) is a synthetic peptide that activates three hormone receptors simultaneously: the GLP-1 receptor (GLP-1R), the glucose-dependent insulinotropic polypeptide receptor (GIPR), and the glucagon receptor (GCGR). This triple-receptor mechanism distinguishes it from semaglutide (GLP-1R only) and tirzepatide (GLP-1R + GIPR). For a full mechanistic overview, see the Retatrutide Research Overview. For head-to-head pharmacological comparisons, see the retatrutide vs tirzepatide vs semaglutide guide.

Velox Peptides supplies retatrutide as an HPLC-verified (≥99% purity) lyophilised reagent for in vitro research use only. View the Retatrutide product page →

How was TRIUMPH-1 designed?

TRIUMPH-1 enrolled 2,339 adults — the largest Phase 3 obesity RCT conducted to date for a GLP-1-class compound. Participants required a body mass index (BMI) of ≥30 kg/m² (obesity) or ≥27 kg/m² with at least one weight-related comorbidity (overweight with comorbidity), and explicitly excluded individuals with type 2 diabetes, preserving a clean non-diabetic metabolic background for the primary analysis.[2]

Participants were randomised 1:1:1:1 to four arms:

• Retatrutide 4 mg once weekly (subcutaneous injection)
• Retatrutide 9 mg once weekly
• Retatrutide 12 mg once weekly
• Placebo

Treatment duration was 80 weeks, followed by a 24-week off-treatment follow-up period for the main cohort. A subset of higher-BMI participants (≥35 kg/m²) continued to a 104-week extended treatment arm. The primary endpoint was percentage change in body weight from baseline to week 80. Pre-specified secondary endpoints included the proportion achieving ≥5%, ≥10%, ≥15%, ≥20%, and ≥30% weight reduction; cardiometabolic markers; and patient-reported outcomes.

TRIUMPH-1 also contained two nested, pre-specified basket trials:

• OSA basket: n=243 participants with moderate-to-severe obstructive sleep apnea at baseline, assessing change in apnoea-hypopnoea index (AHI) — covered in detail in the Retatrutide & Obstructive Sleep Apnea guide.
• OA basket: n=574 participants with knee osteoarthritis, assessing WOMAC pain subscale changes — covered in detail alongside TRIUMPH-4 data in the Retatrutide & Knee Osteoarthritis guide.

What did the primary weight-loss results show?

At 80 weeks, the TRIUMPH-1 primary endpoint was met across all three active doses versus placebo. Participants on retatrutide 12 mg reported a mean body-weight reduction of 28.3% (approximately 70.3 lbs / 32 kg), compared with 2.2% in the placebo arm. The 4 mg and 9 mg doses also significantly exceeded placebo, demonstrating a clear dose-response relationship.[1][3]

The 104-week extended-treatment cohort (BMI ≥35) reported a mean body-weight reduction of up to 30.3% — equivalent to an average of approximately 85 lbs (38.6 kg) per participant. This figure places the pharmacological response squarely within the range historically reported by Roux-en-Y gastric bypass surgery in published surgical series (typically 25–35% total body-weight loss).[3][4]

Dose-response and weight-loss thresholds

The pre-specified responder analysis is particularly informative for receptor pharmacology researchers. At the 12 mg dose, the published figures suggest that nearly all participants on the highest dose exceeded the 5% weight-loss threshold (associated in T2D prevention research with meaningful metabolic improvements), and that the majority crossed the 15% and 20% thresholds associated with remission of obesity comorbidities. The 45.3% achieving ≥30% body weight reduction is a threshold that previous pharmaceutical development regarded as unobtainable pharmacologically — achievable only via surgical intervention.[4]

What secondary outcomes were reported?

Beyond the primary weight endpoint, TRIUMPH-1 reported outcomes from the two pre-specified basket trials embedded within the main study. All data cited below are from the same 80-week trial readout presented at ADA 2026.[1][2]

Obstructive sleep apnea (OSA) basket — n=243

In the 243 participants with moderate-to-severe OSA at baseline, retatrutide 12 mg reported a reduction in the apnoea-hypopnoea index (AHI) of 60.6% (36.1 events per hour) at 80 weeks. This is consistent with the hypothesis that the OSA improvement observed with GLP-1-class compounds is substantially mediated by weight reduction, given the established dose-response between upper-airway fat deposition and OSA severity. Full data and comparison with tirzepatide’s SURMOUNT-OSA dataset are in the dedicated Retatrutide & Obstructive Sleep Apnea guide.

Knee osteoarthritis (OA) basket — n=574

In the 574 participants with knee osteoarthritis, retatrutide 12 mg reported a reduction in WOMAC pain subscale score of 73.1% (4.3 points) at 80 weeks. Complementary data from TRIUMPH-4 (the standalone OA-focused Phase 3 trial, December 2025, n=445) reported 75.8% pain reduction at 68 weeks alongside 28.7% body-weight reduction. The convergent signal across two independent trials strengthens the mechanistic research interest in GLP-1-class compounds and joint-tissue outcomes. Full analysis in the Retatrutide & Knee Osteoarthritis guide.

Safety profile

The tolerability profile reported in TRIUMPH-1 was described as broadly consistent with GLP-1-class compounds. Gastrointestinal adverse events — predominantly nausea, vomiting, and diarrhoea — were the most commonly reported events, and were generally characterised as mild-to-moderate in severity. No unexpected safety signals were described in the Eli Lilly press releases or ADA presentations reviewed for this summary. The treatment duration of 80 weeks provides a substantially longer safety observation window than the Phase 2 NEJM data (48 weeks), which is of interest for researchers studying long-term receptor-level pharmacology.[5]

Why does retatrutide produce greater weight loss than previous GLP-1-class agents?

From a receptor-level research perspective, the TRIUMPH-1 result invites comparison with Phase 3 data from semaglutide (STEP 1: −14.9% at 68 weeks, NEJM 2021) and tirzepatide (SURMOUNT-1: −20.9% at 72 weeks, NEJM 2022). The incremental weight loss with retatrutide is generally attributed in the preclinical literature to the addition of glucagon receptor (GCGR) agonism on top of GLP-1R and GIPR activation.[6]

Glucagon receptor activation contributes two complementary mechanisms not present in GLP-1-only or dual GIP/GLP-1 compounds:

• Increased basal metabolic rate: glucagon stimulates hepatic glycogenolysis and promotes lipolysis in adipose tissue, increasing thermogenic activity independent of food intake.
• Enhanced hepatic fat clearance: GCGR agonism reduces hepatic lipid synthesis and promotes fatty-acid oxidation, a pathway relevant to MASH (metabolic dysfunction-associated steatohepatitis) research as well as metabolic-obesity models.

The preclinical characterisation by Coskun et al. (Cell Metabolism, 2022) demonstrated that these three receptor activities are engineered to act in concert: GIP and GLP-1 agonism moderates the glycaemic risk that pure glucagon agonism would introduce, while glucagon agonism amplifies the weight and lipid effects beyond what dual-agonism alone achieves.[6] The TRIUMPH-1 data provide the first large-scale Phase 3 confirmation of this preclinical hypothesis in a human obesity model. For the Phase 1 and Phase 2 clinical research background underpinning this, see the Retatrutide Clinical Research guide.

Where does TRIUMPH-1 fit in the broader TRIUMPH programme?

TRIUMPH-1 is the primary obesity registrational trial, but it is one of eight pivotal trials in Eli Lilly’s TRIUMPH programme, collectively enrolling more than 5,800 participants across weight-management and diabetes indications. A separate cardiovascular outcomes trial (CVOT) is enrolling approximately 10,000 patients.[7]

Note: cross-trial comparisons must be interpreted with caution. Differences in trial duration, population, design, and primary endpoints preclude direct mechanistic conclusions. Table shown for contextual research reference only.

Velox Peptides Supply Information

Retatrutide is supplied as a research reagent only. It is not a medicine and has not been evaluated by the MHRA or FDA. Not for human or veterinary use. See our Research Use Policy and MHRA Statement.

Frequently Asked Questions