Published: June 08, 2026, 06:03 UTC

Retatrutide delivers the largest weight loss ever seen in a Phase 3 obesity trial

The race for the next generation of metabolic medicines delivered its most striking result yet this week at the American Diabetes Association’s 86th Scientific Sessions in New Orleans. Eli Lilly’s retatrutide, a once-weekly injection that activates three separate hormone receptors, produced weight loss and blood sugar reductions that surpass anything previously seen in a Phase 3 trial. The results were published simultaneously in The Lancet on June 6.

Retatrutide, also known by its development code LY3437943, belongs to a new class of drugs sometimes called “triple G” agonists. It targets three receptors at once: GLP-1 (glucagon-like peptide-1), GIP (glucose-dependent insulinotropic polypeptide), and glucagon. Existing drugs such as semaglutide (Ozempic/Wegovy) target GLP-1 alone, while tirzepatide (Mounjaro/Zepbound) targets both GLP-1 and GIP. Adding glucagon agonism, which increases energy expenditure, is the key theoretical advantage that could push weight loss beyond the roughly 20 to 22 percent seen with the best current options.

Two Phase 3 trials were presented at ADA 2026: TRANSCEND-T2D-1 in type 2 diabetes and TRIUMPH-1 in obesity. Both met their primary endpoints with statistical significance and effect sizes that reset expectations for what a metabolic drug can achieve.

TRANSCEND-T2D-1: Diabetes control

The diabetes trial enrolled 537 adults with type 2 diabetes inadequately controlled by diet and exercise alone, with HbA1c between 7.0 and 9.5 percent and a mean BMI of 35.8. Participants were randomized to one of three doses of retatrutide (4 mg, 9 mg, or 12 mg) or placebo, with dosing escalated stepwise over several weeks to improve tolerability.

At 40 weeks, the results were as follows:

The 12 mg dose produced a mean HbA1c reduction of 1.94 percentage points, from a baseline of 7.9 percent to a final value of approximately 5.9 percent. The 9 mg dose produced a 1.86-point reduction, and the 4 mg dose produced a 1.69-point reduction. All were statistically significant versus placebo, which saw a 0.81-point reduction.
Across the retatrutide groups, 35 to 46 percent of participants achieved a normal HbA1c of below 5.7 percent, a state effectively indistinguishable from non-diabetic glucose regulation. Up to 90 percent reached the standard ADA target of HbA1c below 7.0 percent.
Weight loss followed a clear dose-response curve. The 12 mg group lost 15.3 percent of body weight at 40 weeks, equivalent to roughly 16.6 kg (36.6 lb). The 9 mg group lost 13.9 percent, and the 4 mg group lost 11.5 percent. Critically, weight loss had not plateaued by week 40, suggesting further losses may be possible with longer treatment.

Fasting glucose fell by approximately 31 mg/dL across the retatrutide groups, compared with just 1 mg/dL in the placebo arm.

The lead investigator, Harpreet S. Bajaj of LMC Diabetes and Endocrinology in Toronto, described the results as evidence that triple agonism can deliver meaningful advantages over dual and single agonists, particularly for weight reduction in a diabetes population.

TRIUMPH-1: Obesity

The obesity trial was larger and longer. It enrolled 2,339 adults with obesity or overweight, randomizing them to retatrutide or placebo for 80 weeks, with a 104-week extension in a subset of participants.

At 80 weeks, the 12 mg dose produced a 28.3 percent mean weight loss (25.9 percent using a more conservative efficacy estimand), or approximately 31.9 kg (70.3 lb). The 9 mg group lost 25.9 percent, and the 4 mg group lost 19.0 percent. The placebo group lost 3.9 percent.

A closer look at the distribution of responses reveals just how potent the drug is at the individual level:

87.5 percent of participants in the 12 mg group lost at least 15 percent of their body weight.
45.3 percent lost at least 30 percent.
27.2 percent lost at least 35 percent.
65.3 percent achieved a BMI below 30, moving out of the obesity range.
33.3 percent reached a BMI below 25, classified as normal weight.

At the two-year mark in the extension cohort, participants on the 12 mg dose maintained a 29.9 percent weight loss, with some individuals losing over 38.6 kg (85 lb).

Ania M. Jastreboff, director of the Yale Obesity Research Center and lead investigator of TRIUMPH-1, noted that the magnitude of weight loss approaches what is typically seen with bariatric surgery, making retatrutide the first medical therapy to reach this threshold in a Phase 3 trial.

The glucagon advantage

One of the theoretical concerns about activating the glucagon receptor in a diabetes drug is the risk of raising blood glucose. Glucagon’s traditional role is to increase blood sugar during fasting, so adding it to a once-weekly injection for people with diabetes seemed counterintuitive. The trial data show that this concern did not materialize: glucose continued to drop, likely because the GLP-1 and GIP components of the drug more than compensate for any glucagon-driven glucose production.

The glucagon component instead appears to drive additional weight loss through increased energy expenditure, a mechanism distinct from the appetite suppression that GLP-1 and GIP provide. This dual pathway — reduced calorie intake plus increased calorie burning — is the reason retatrutide’s weight loss numbers exceed those of tirzepatide by a meaningful margin.

Side effects and tolerability

The side effect profile of retatrutide was consistent with other GLP-1-based therapies, predominantly gastrointestinal. Nausea affected 29 to 42 percent of participants in the obesity trial, diarrhea 25 to 32 percent, and vomiting 11 to 25 percent. Constipation was also common at 24 to 26 percent. These rates are broadly comparable to tirzepatide at equivalent therapeutic doses.

Discontinuation due to adverse events reached 11.3 percent in the 12 mg group of the obesity trial, higher than the 4.1 and 6.9 percent seen at the lower doses, and notably above the 4.9 percent discontinuation rate in the placebo group. This suggests that the top dose, while the most effective, pushes against the limits of tolerability for some patients.

Two new safety signals emerged that had not been prominent in earlier trials of retatrutide or other GLP-1 drugs. Urinary tract infections were reported in 6.8 to 8.1 percent of retatrutide-treated participants in the obesity trial, predominantly in women (92 percent of cases). Dysesthesia, a sensation of abnormal touch or prickling, occurred in 4.4 percent of participants. Both require further investigation in longer-term studies.

Serious adverse events occurred in 7.7 to 10.5 percent of retatrutide-treated participants across the two trials, compared with 5.5 percent in the placebo arm. Deaths were reported in the treatment groups but were not deemed by investigators to be related to the study drug.

Where retatrutide fits

TRANSCEND-T2D-1 enrolled only people who were not taking any diabetes medications and had relatively short disease duration (mean 2.5 years). This limits direct comparison with trials that included patients on background therapies. Retatrutide is also being tested against semaglutide in the ongoing TRANSCEND-T2D-2 trial, and head-to-head data will clarify whether its advantages are truly transformative or incremental.

An accompanying commentary in The Lancet by Ildiko Lingvay and Shuyao Zhang of UT Southwestern noted that retatrutide’s principal advantage appears to be in weight reduction rather than glycemic control. The HbA1c reduction of approximately 2 percentage points is similar to what tirzepatide achieves. The differentiation comes from the magnitude of weight loss, which is roughly 5 to 8 percentage points greater than tirzepatide’s best results.

Retatrutide has not yet been submitted for regulatory approval. Eli Lilly has a comprehensive Phase 3 program underway, including cardiovascular outcomes trials that will be essential for determining the drug’s long-term safety and its place in the treatment hierarchy. Cardiovascular outcomes data are expected within the next two to three years, and regulatory submissions are anticipated later in 2026 or early 2027.

References:

Bajaj HS, et al. “Efficacy and safety of retatrutide, a GIP, GLP-1, and glucagon receptor agonist, for people with type 2 diabetes inadequately controlled by diet and exercise (TRANSCEND-T2D-1): a double-blind, randomised, phase 3 trial.” The Lancet, 6 June 2026. DOI: 10.1016/S0140-6736(26)00967-0

Jastreboff AM, et al. “TRIUMPH-1: Phase 3 trial of retatrutide in obesity.” Presented at ADA 86th Scientific Sessions, New Orleans, June 2026.

Zhang S, Lingvay I. “Multireceptor modulation in metabolic disease: are more targets better?” The Lancet, 6 June 2026. DOI: 10.1016/S0140-6736(26)01136-0