Retatrutide explained: triple agonist, Phase 3 results and approval status

What retatrutide is

Retatrutide (development code LY3437943) is an investigational once-weekly injectable molecule developed by Eli Lilly. It activates three hormone receptors: GIP, GLP-1 and glucagon. That is why it is often called a triple agonist.

For comparison, semaglutide acts on one of these receptors (GLP-1) and tirzepatide on two (GIP and GLP-1). The added glucagon activity is thought to increase energy expenditure and affect liver fat, but the clinical relevance of each component is still being studied.

Phase 2: the 2023 NEJM results

The first large efficacy data came from a 48-week Phase 2 trial in adults with obesity, published in the New England Journal of Medicine in 2023. Average weight change was −8.7% in the 1 mg group, −17.1% in the 4 mg groups, −22.8% in the 8 mg groups and −24.2% in the 12 mg group.

Those numbers attracted attention because they exceeded what had been reported for any single approved weight-management medicine at that time. Phase 2 trials are smaller and shorter than registrational studies, so the field waited for Phase 3.

Phase 3: TRIUMPH-4, TRIUMPH-1 and TRIUMPH-2

TRIUMPH-4 (topline December 2025) studied people with obesity and knee osteoarthritis. Participants on 12 mg lost an average of 28.7% of body weight at 68 weeks, and knee-pain scores improved substantially.

TRIUMPH-1 (topline May 2026) is the pivotal obesity trial. In 2,339 adults with obesity or overweight and a weight-related condition, the 12 mg dose produced an average 28.3% weight loss at 80 weeks, and 45.3% of participants lost at least 30% of their body weight. The 4 mg dose produced 19.0%.

TRIUMPH-2 studied people with obesity and type 2 diabetes. Average weight loss on 12 mg was 20.8% at 80 weeks. Lower weight loss in people with diabetes is a pattern also seen with other incretin medicines.

These are company-reported results; full peer-reviewed publications and regulatory review are what ultimately determine an approved label.

Retatrutide vs tirzepatide vs semaglutide

There is no published head-to-head trial of retatrutide against tirzepatide or semaglutide. Comparing 28.3% in TRIUMPH-1 with 20.9% in SURMOUNT-1 or 14.9% in STEP 1 is tempting, but those trials differed in duration, population and design.

What can be said: retatrutide produced the largest average weight loss reported so far in a Phase 3 obesity programme, and its side-effect profile in Lilly's reports has been described as consistent with incretin-based therapies, mainly gastrointestinal.

Is retatrutide approved?

No. Lilly states that retatrutide is an investigational molecule available only to participants in its clinical trials and has not been approved by any regulatory agency. Check the regulator in your country for current status; filing timelines reported in the press are not approvals.

In August 2026, Lilly Canada issued a public safety notice about unapproved and illegal retatrutide sold on the black market. In March 2026 the FDA sent a warning letter to a US website offering retatrutide. Products sold under the retatrutide name outside clinical trials are not the material tested in TRIUMPH, and their identity, purity and sterility are unknown unless independently documented.

How to read retatrutide claims online

Treat any retatrutide claim as one of three things: trial data for Lilly's investigational product, speculation about future approval, or marketing for a product of unknown origin. Only the first is evidence, and it applies only to the trial material.

If you are evaluating a research-material listing, start with the documentation: batch number, independent identity testing, purity method, and what was not tested. Our certificate-of-analysis checklist explains each item.