retatrutide (LY3437943): the triple GIP/GLP-1/glucagon agonist in phase 3

retatrutide (development code LY3437943) is an investigational peptide from Eli Lilly that activates three hormone receptors simultaneously: the GIP receptor, the GLP-1 receptor, and the glucagon receptor. in phase 2 trials it produced the largest mean weight-loss signal seen in any incretin peptide to date. it is not FDA-approved for any indication, cannot legally be compounded in the US, and all current use should be in the context of formal clinical trials.

what is retatrutide and why add glucagon?

retatrutide builds on the dual GIP/GLP-1 framework established by tirzepatide by adding glucagon receptor agonism as a third axis. glucagon is typically associated with raising blood sugar, but at the receptor and tissue level it also drives significant increases in energy expenditure and fat oxidation, particularly in the liver. the hypothesis behind triple agonism is that adding this energy-expenditure component on top of the appetite and glycemic effects of dual agonism could push the weight-loss ceiling substantially higher.

the GLP-1 receptor component drives appetite suppression, glucose-dependent insulin secretion, glucagon suppression, and satiety signaling in the central nervous system. the GIP receptor component adds complementary incretin effects on beta-cell function, adipose tissue insulin sensitivity, and central appetite pathways. the glucagon receptor component is intended to increase hepatic glucose output in the fasting state (which requires careful balance in a therapeutic context), stimulate fatty acid oxidation, and increase energy expenditure through thermogenic pathways [1]. the relative contribution of each receptor to net clinical outcomes is still being characterized as phase 3 data matures; mechanistic partitioning at this stage remains inferential.

what do the phase 2 trials show?

two peer-reviewed phase 2 publications and a phase 2a liver substudy provide the current evidence base. the obesity trial (NEJM 2023) reported approximately 24% mean weight loss at 48 weeks at the highest dose, the type 2 diabetes trial (Lancet 2023) showed dose-dependent HbA1c and weight reduction, and the MASLD substudy (Nature Medicine 2024) reported striking liver-fat reductions at higher doses.

the phase 2 obesity trial (NCT04881760, n=338) randomized adults without type 2 diabetes across multiple dose and escalation arms. mean weight change at 24 weeks reached approximately -17.5% in the 12 mg arm versus -1.6% for placebo. at 48 weeks, the 12 mg arm reached approximately -24.2% mean weight loss versus -2.1% for placebo [2]. the most common adverse events were gastrointestinal and dose-related, with most events mild to moderate. a notable finding was a dose-dependent heart-rate increase that peaked around week 24 and then declined -- a signal not seen with the dual agonists, plausibly related to glucagon receptor-mediated cardiac effects, and a priority monitoring item for the ongoing phase 3 program.

the phase 2 type 2 diabetes trial (NCT04867785, Lancet 2023, n=338) showed dose-dependent HbA1c improvements reaching approximately -2.0 percentage points in high-dose escalation arms at 24 weeks, with body weight also falling dose-dependently into the mid-to-high-teen percentage range by 36 weeks [3]. no severe hypoglycemia and no deaths were reported in either phase 2 publication.

a phase 2a MASLD substudy (Nature Medicine 2024) examined liver-fat changes in participants from the obesity trial. strong relative liver-fat reductions were reported at 24 weeks in higher-dose arms, with large proportions reaching normal liver-fat thresholds below 5%, compared with none in the placebo group in the reported analysis [4]. this is a promising hepatic signal with biological plausibility from both the weight-loss and glucagon-mediated hepatic fat-oxidation mechanisms, but hard liver outcomes and long-term fibrosis endpoints remain under investigation.

the TRIUMPH phase 3 program and what it will answer

the TRIUMPH and TRANSCEND programs span obesity without diabetes, type 2 diabetes, obesity with cardiovascular disease, obesity with osteoarthritis, and head-to-head comparison with tirzepatide. TRIUMPH-4, the obesity with knee osteoarthritis trial, reported positive topline results in December 2025. most decisive long-term comparative and outcomes data are still in progress.

TRIUMPH-4 (December 2025 topline) evaluated retatrutide in adults with obesity or overweight and knee osteoarthritis without diabetes. Lilly's December 2025 summary reported co-primary endpoints met at 68 weeks in both the 9 mg and 12 mg groups, with up to approximately 24.3% mean body-weight reduction and substantial WOMAC pain-score improvement versus placebo [5]. the full peer-reviewed manuscript and complete statistical details were not yet published as of mid-2026, so this topline should be treated as interim confidence pending full publication.

TRIUMPH-5 (NCT06662383) is a direct head-to-head comparison of retatrutide versus tirzepatide, which will provide the first controlled head-to-head efficacy and safety comparison between the dual and triple agonist strategies. TRIUMPH-Outcomes (NCT06383390) is the event-driven cardiovascular and kidney outcomes trial, with estimated primary completion in 2029 [6]. additional programs (TRIUMPH-1, TRIUMPH-2, TRIUMPH-3, TRANSCEND T2D-1, TRANSCEND T2D-3) are evaluating obesity without diabetes, type 2 diabetes, obesity with cardiovascular disease, and type 2 diabetes with renal impairment respectively.

regulatory status and access

retatrutide is not FDA-approved for any indication as of 2026. FDA has stated it cannot be used in compounding under federal law, and any commercially sold product labeled as retatrutide represents an illegally marketed unapproved drug. the only legitimate access is participation in a registered clinical trial.

the FDA has issued warning actions related to illegally marketed products containing GLP-1 class peptides including retatrutide, and warns against products sold as "research use only" but promoted for human use. unlike semaglutide and tirzepatide, which had compounding-pharmacy availability during shortage periods, retatrutide has never had an approved commercial product and thus has never had a legitimate shortage-based compounding pathway. the regulatory timeline for potential approval depends on the completion of the TRIUMPH phase 3 program and subsequent NDA submission and FDA review, a timeline that extends into 2027-2028 at the earliest based on current trial schedules.

where retatrutide fits in the incretin landscape

retatrutide represents the next step beyond tirzepatide in the incretin evolution: from GLP-1-only (semaglutide) to dual GIP/GLP-1 (tirzepatide) to triple GIP/GLP-1/glucagon. if the phase 3 data confirm the phase 2 efficacy signal, it would represent the highest weight-loss efficacy of any approved incretin medicine. the key unknowns are the long-term cardiovascular safety of glucagon receptor activation at therapeutic doses and whether the heart-rate signal seen in phase 2 resolves or requires management in longer-term use.

tirzepatide currently has the strongest approved evidence and the head-to-head superiority over semaglutide from SURMOUNT-5, but its weight-loss ceiling (~22.5% in SURMOUNT-1) is lower than what the retatrutide phase 2 data suggest is achievable with triple agonism. survodutide (Boehringer Ingelheim) is a different dual GLP-1/glucagon agonist in phase 3 with a particular focus on liver disease; it takes a different combination than retatrutide and may carve out a distinct niche. for context on how GH-axis peptides like tesamorelin interact with this incretin landscape, the underlying biology is covered in our free peptides and your body module.