Retatrutide: What It Is and Where the Research Stands

Approved medicines already target GLP-1 alone or GLP-1 and GIP together. Retatrutide adds glucagon-receptor activity. That scientific difference is important, but having three targets does not automatically make a medicine safer, more effective or better suited to an individual.

Only completed clinical trials and regulatory review can establish its benefit-risk profile for a particular use.

Early clinical development#

Retatrutide was initially known by its development code, LY3437943. Early studies evaluated its effects on blood glucose, body weight, tolerability and pharmacology in people with and without type 2 diabetes.

Phase 2 results in 2023#

A randomized Phase 2 obesity trial published in the New England Journal of Medicine in 2023 brought retatrutide broad attention. In the highest-dose trial group, mean weight change reached 24.2% after 48 weeks. The result was an average from a controlled research population, not a forecast for an individual.

A separate Phase 2 study in people with type 2 diabetes examined blood-glucose control and weight change. These studies supported further development but were not sufficient for approval.

Expansion into Phase 3#

Lilly launched a broad Phase 3 programme covering obesity, type 2 diabetes and several related conditions. Registered studies have examined or are examining knee osteoarthritis pain, sleep apnoea, cardiovascular and kidney outcomes, liver disease and chronic low-back pain in defined populations.

Phase 3 evidence in 2026#

In June 2026, The Lancet published TRANSCEND-T2D-1, a 40-week Phase 3 trial involving 537 adults whose type 2 diabetes was inadequately controlled with diet and exercise alone. Retatrutide groups had larger average reductions in HbA1c and body weight than placebo. The study was funded by Lilly, and several authors were company employees or shareholders.

On 21 May 2026, Lilly announced positive topline findings from TRIUMPH-1, its Phase 3 obesity trial. The company highlighted average weight reduction of up to 28.3% at 80 weeks in one prespecified analysis of the highest-dose group. Another reported analysis produced a lower estimate. Full interpretation requires the complete methods, all endpoints and peer-reviewed publication.

Obesity and overweight#

The largest public interest concerns weight management. Phase 2 research and sponsor-reported Phase 3 data show substantial average weight reductions in selected trial populations receiving structured follow-up.

Those figures should not be compared directly with percentages from unrelated trials of other medicines. Differences in participant criteria, duration, analysis methods, adherence and missing-data handling can change the result.

Type 2 diabetes#

TRANSCEND-T2D-1 provides peer-reviewed Phase 3 evidence in adults with type 2 diabetes inadequately controlled by diet and exercise. At 40 weeks, mean HbA1c reductions ranged from 1.69 to 1.94 percentage points across retatrutide groups, compared with 0.81 points with placebo. Mean weight changes ranged from 11.5% to 15.3%, compared with 2.6% with placebo.

These results apply to the studied population and trial conditions. Retatrutide still requires regulatory review before it can become an approved diabetes treatment.

Weight-related joint symptoms#

TRIUMPH-4 studied adults with obesity or overweight and knee osteoarthritis. Weight change and knee-pain measures were co-primary outcomes. The trial helps researchers test whether substantial weight reduction is accompanied by improvements in symptoms and physical function; it does not establish retatrutide as a general pain medicine.

Other cardiometabolic conditions#

Lilly's Phase 3 programme includes defined studies involving obstructive sleep apnoea, cardiovascular and kidney outcomes, metabolic liver disease and chronic low-back pain. Inclusion in a development programme means a question is being tested. It does not mean the medicine has been proven or approved for that condition.

Phase 2 obesity trial#

The 2023 randomized, double-blind trial included adults with obesity, or overweight plus at least one weight-related condition, without diabetes. At 48 weeks, the highest-dose group had a mean weight reduction of 24.2%.

Gastrointestinal adverse events were the most common. They occurred mainly while doses were being increased, were generally mild to moderate and were more frequent in higher-dose groups. Heart rate increased in a dose-dependent pattern up to 24 weeks and then declined later in the trial.

TRANSCEND-T2D-1 Phase 3 diabetes trial#

The 2026 trial randomized 537 adults to three retatrutide groups or placebo for 40 weeks. Retatrutide produced larger mean improvements in HbA1c and body weight. The most frequent adverse events were gastrointestinal and generally mild to moderate. Treatment discontinuation because of adverse events occurred in 2% to 5% of retatrutide participants and 0% of placebo participants. No severe hypoglycaemia was reported.

Two deaths occurred in one retatrutide group; the investigators reported that both were unrelated to the study drug. Reporting that assessment does not remove the need for broader safety evaluation.

TRIUMPH-1 Phase 3 obesity announcement#

Lilly's May 2026 announcement described positive results from 2,339 participants studied for 80 weeks. The company reported substantial average weight reductions across all retatrutide groups.

The most common events reported by Lilly were nausea, diarrhoea, constipation and vomiting. Dysesthesia—altered or unusual sensation—was also reported more often with retatrutide than placebo. Discontinuation due to adverse events increased across the reported dose groups.

These are sponsor-reported topline results. Detailed presentation and peer-reviewed publication are needed for independent evaluation of the full dataset.

United States#

Retatrutide is not FDA approved. The FDA states that retatrutide is not a component of an FDA-approved drug and has not been found safe and effective for any condition.

The FDA also states that retatrutide cannot be used in compounding under US federal law. It is not eligible merely because a seller calls it "research use only," "custom," or a compounded GLP-1 product.

Australia#

Retatrutide is not included in the Australian Register of Therapeutic Goods as an approved medicine. On 19 June 2026, the TGA and Australia's Chief Medical Officer identified retatrutide among unapproved peptide products of public-health concern.

The TGA warning emphasises that unapproved products have not been evaluated for safety, quality or effectiveness and may have uncertain ingredients, labelling, manufacturing conditions and sterility.

Clinical-trial access#

Lilly states that retatrutide is legally available only to participants in its clinical trials. A product sold online under the same name is not equivalent to participation in a regulated trial, where product quality, eligibility, monitoring and adverse-event reporting are controlled.

The frequency and severity varied by trial group and study. Trial findings do not establish that an unapproved online product has the same composition, purity or risk profile as Lilly's investigational medicine.

Important uncertainties remain:

• rare adverse effects that require larger or longer studies to detect;
• long-term cardiovascular and kidney outcomes;
• durability after treatment is stopped;
• safety in populations not represented in the trials;
• interactions and contraindications that regulators may ultimately place in approved labelling;
• whether every proposed indication will show a favourable benefit-risk balance.