Frequently Asked Questions

Retatrutide (LY3437943) is an investigational medication developed by Eli Lilly and Company. It is a single peptide molecule that simultaneously activates three metabolic hormone receptors: GIP (glucose-dependent insulinotropic polypeptide), GLP-1 (glucagon-like peptide-1), and glucagon receptors. This triple receptor agonism is a novel approach designed to address obesity and type 2 diabetes through complementary metabolic pathways.

No. As of March 2026, retatrutide is an investigational compound in Phase 3 clinical development. It has not been approved by the FDA or any other regulatory authority worldwide. It is not available for prescription or commercial purchase. Any legitimate use is limited to authorized clinical trials.

Retatrutide is being developed by Eli Lilly and Company, a major pharmaceutical company based in Indianapolis, Indiana. Lilly also developed tirzepatide (marketed as Mounjaro for type 2 diabetes and Zepbound for obesity), which is a related dual GIP/GLP-1 receptor agonist.

LY3437943 is retatrutide's development code assigned by Eli Lilly. The 'LY' prefix identifies it as a Lilly compound, and the numerical designation is its internal identifier. 'Retatrutide' is the drug's International Nonproprietary Name (INN), which is the standardized name for scientific and medical communication.

Retatrutide is primarily being studied for the treatment of obesity and weight management in the Phase 3 TRIUMPH program (4 trials: TRIUMPH-1 through TRIUMPH-4, plus TRIUMPH-Outcomes for cardiovascular events). The separate TRANSCEND program (TRANSCEND-T2D-1, T2D-2, T2D-3, CKD) evaluates retatrutide for type 2 diabetes. It is also being evaluated for metabolic dysfunction-associated steatotic liver disease (MASLD, formerly NAFLD) in dedicated trials: the SYNERGY trial and the MACELD Phase 3 trial, which launched in Q4 2025. The Phase 2 data showed significant liver fat reduction with 86% of participants at the 12 mg dose achieving normal liver fat levels, supporting the dedicated liver disease program.

Semaglutide (Ozempic/Wegovy) targets only the GLP-1 receptor. Tirzepatide (Mounjaro/Zepbound) targets both GIP and GLP-1 receptors. Retatrutide targets all three: GIP, GLP-1, and glucagon receptors. The addition of glucagon receptor agonism is hypothesized to increase energy expenditure and promote liver fat reduction, which are effects not available from single or dual agonists.

No. Retatrutide does not have a brand name because it has not received marketing authorization from any regulatory agency. Brand names are typically assigned during the regulatory approval process. The drug is referred to by its INN (retatrutide) or development code (LY3437943) in scientific literature.

Eli Lilly indicated during its Q4 2025 earnings call (February 4, 2026) that NDA filing for retatrutide is expected in late 2026. Based on this guidance, FDA approval could come as early as mid-2027 (if Priority Review is granted) or late 2027 to early 2028 under standard review. These are estimates and depend on Phase 3 trial outcomes and regulatory decisions.

A triple agonist is a single molecule that activates three different receptor types. Retatrutide activates GIP receptors, GLP-1 receptors, and glucagon receptors. Each receptor triggers different biological effects, and their simultaneous activation produces complementary metabolic benefits that are hypothesized to be greater than any single receptor pathway alone.

GLP-1 receptor activation reduces appetite through effects on brain satiety centers, stimulates insulin secretion in a glucose-dependent manner (lowering blood sugar without causing dangerous drops), suppresses glucagon release from the pancreas, and slows gastric emptying. This is the same mechanism used by approved drugs like semaglutide (Ozempic/Wegovy) and liraglutide (Saxenda).

GIP receptor activation enhances insulin secretion, influences adipose (fat) tissue function, and may contribute to appetite suppression through brain pathways. When combined with GLP-1 receptor activation, GIP appears to amplify the metabolic benefits, as demonstrated by tirzepatide (Mounjaro/Zepbound), which targets both GIP and GLP-1 receptors.

Glucagon receptor activation increases energy expenditure (the number of calories the body burns at rest) and promotes fatty acid oxidation in the liver (burning stored fat for energy). This component is unique to retatrutide among advanced clinical candidates and is thought to explain the enhanced weight loss and dramatic liver fat reduction observed in clinical trials.

Glucagon naturally raises blood glucose by stimulating the liver to release stored sugar. In retatrutide, this glucose-raising effect is counterbalanced by the simultaneous GLP-1 and GIP receptor activation, which promote insulin secretion and suppress excessive glucagon signaling. Clinical trial data confirm that the net effect is improved, not worsened, blood sugar control.

Retatrutide's peptide structure includes a fatty acid modification that allows it to bind to albumin, a protein in the blood. This albumin binding protects the drug from being broken down quickly and extends its half-life to approximately six days, meaning that a single injection provides sustained drug levels throughout a full week.

Retatrutide promotes weight loss through two complementary mechanisms: reducing caloric intake (through appetite suppression mediated by GLP-1 and GIP receptor activation in the brain) and increasing caloric expenditure (through energy expenditure increases driven by glucagon receptor activation). This dual approach to the energy balance equation distinguishes it from single- and dual-agonist therapies that primarily reduce intake.

As of March 2026, retatrutide has completed Phase 1, Phase 2, and the first Phase 3 trial (TRIUMPH-4). The Phase 2 program included a 48-week obesity trial (NCT04881706, published in the New England Journal of Medicine) and a 36-week type 2 diabetes trial (NCT04867785, published in The Lancet). TRIUMPH-4 Phase 3 topline results were announced in December 2025 showing 28.7% weight loss at 12 mg over 68 weeks. Both Phase 2 trials produced positive results that supported advancement to Phase 3.

The Phase 2 obesity trial demonstrated mean weight loss of up to 24.2% at 48 weeks in the highest dose group (12 mg), with 100% of participants at that dose losing at least 5% of body weight. The trial also showed an approximately 82% relative reduction in liver fat, improvements in blood pressure and lipids, and a manageable gastrointestinal side effect profile. These were the highest weight loss results ever reported for a pharmacological agent.

The Phase 2 type 2 diabetes trial showed HbA1c reductions of up to 2.02 percentage points at 36 weeks in the highest dose group. Approximately 95% of participants at the highest dose achieved an HbA1c below 7.0% (the standard treatment target), and approximately 62% reached HbA1c below 5.7% (the non-diabetic range). Retatrutide at higher doses significantly outperformed the active comparator, dulaglutide 1.5 mg.

TRIUMPH (Triple Receptor Agonist Intervention Utilizing Metabolic Pathways in Health) is the Phase 3 clinical development program for retatrutide focused on obesity and weight management. It includes 4 numbered trials: TRIUMPH-1 (obesity/overweight without T2D, with nested OSA and OA baskets), TRIUMPH-2 (obesity/overweight with T2D, with nested OSA basket), TRIUMPH-3 (Class II/III obesity with established cardiovascular disease), and TRIUMPH-4 (obesity with knee osteoarthritis, completed December 2025). TRIUMPH-Outcomes is a separate 10,000-patient cardiovascular outcomes trial. The separate TRANSCEND program addresses the standalone T2D indication.

TRIUMPH-4 reported topline results in December 2025 (the first Phase 3 readout). Per Eli Lilly's Q4 2025 earnings call (February 2026), 6 additional Phase 3 readouts are expected in 2026, and Lilly plans to file a New Drug Application (NDA) with the FDA in 2026. The core obesity (TRIUMPH-1) and diabetes (TRIUMPH-2) results are among those expected in 2026.

This resource does not facilitate clinical trial enrollment. If you are interested in participating in a clinical trial, discuss this with your healthcare provider or search for registered studies on ClinicalTrials.gov using the search term 'retatrutide' or 'LY3437943'. Eligibility criteria vary by trial and are determined by the study investigators.

In Phase 2 clinical trials, the most commonly reported side effects were gastrointestinal in nature: nausea, diarrhea, vomiting, constipation, and decreased appetite. These effects are consistent with the known pharmacology of GLP-1 receptor agonists and were predominantly mild to moderate in severity. Most GI events occurred during the dose-escalation period and diminished over time as participants adapted to the medication.

In the Phase 2 obesity trial, treatment discontinuation due to adverse events ranged from approximately 0% at the lowest dose (1 mg) to approximately 16% at the highest dose (12 mg). Phase 3 TRIUMPH-4 data showed discontinuation rates of 4.0% with placebo, 12.2% with 9 mg, and 18.2% with 12 mg. These rates were driven primarily by gastrointestinal intolerance and are within the range observed for other incretin-based therapies. Gradual dose escalation was shown to improve tolerability compared to fixed-dose starts.

In the Phase 2 obesity trial (participants without diabetes), no significant hypoglycemia was reported. In the type 2 diabetes trial, hypoglycemia rates were low and were primarily associated with concomitant use of other diabetes medications (such as sulfonylureas) that can independently cause low blood sugar. Retatrutide's insulin-promoting effects are glucose-dependent, meaning they are activated mainly when blood sugar is elevated, which inherently reduces hypoglycemia risk.

Retatrutide was associated with a modest increase in resting heart rate (approximately 2-4 beats per minute), which is a known class effect of GLP-1 receptor agonists. Blood pressure and lipid profiles improved with treatment, which are favorable cardiovascular signals. However, no dedicated cardiovascular outcomes trial has been completed for retatrutide, so definitive cardiovascular safety or benefit data are not yet available.

The longest published treatment duration for retatrutide is 68 weeks from the Phase 3 TRIUMPH-4 trial (December 2025). Long-term safety data over multiple years, which are essential for a medication intended for chronic use, will be generated by the broader Phase 3 TRIUMPH program and post-marketing surveillance if the drug is approved. Long-term effects on bone density, muscle mass, nutritional status, and organ function require continued study.

No unexpected serious safety signals were reported in the Phase 2 retatrutide trials. However, Phase 3 TRIUMPH-4 data (December 2025) identified a new signal: dysesthesia — abnormal sensations including tingling, numbness, or burning — reported in 8.8% of the 9 mg group and 20.9% of the 12 mg group, compared with 0.7% for placebo. This signal was not prominent in Phase 2 data and is thought to be linked to glucagon receptor activity. Dysesthesia was generally mild and infrequently led to discontinuation. The broader safety profile remains consistent with the known effects of incretin-based therapies, though Phase 3 trials with larger populations continue to provide a more comprehensive safety assessment.

Retatrutide is not currently available for prescription or purchase. It is in Phase 3 clinical development. Eli Lilly indicated during its Q4 2025 earnings call (February 4, 2026) that NDA filing is expected in late 2026. Based on this guidance, FDA approval could come as early as mid-2027 (if Priority Review is granted) or late 2027 to early 2028 under standard review. These are estimates and are subject to change based on trial outcomes and regulatory processes.

The only legitimate way to receive retatrutide currently is through participation in an authorized clinical trial. The drug is not approved for prescription, and purchasing it outside of a clinical trial setting is not sanctioned by regulatory authorities. We strongly advise against obtaining any investigational compound through unregulated channels.

In all clinical trials to date, retatrutide has been administered as a once-weekly subcutaneous injection. If approved, it is expected to be available initially as an injectable formulation. Whether an oral formulation might be developed in the future is unknown and would require separate clinical development.

Pricing has not been determined, as retatrutide has not been approved for marketing. For context, other incretin-based obesity medications (semaglutide, tirzepatide) carry list prices in the range of approximately $1,000-$1,300 per month in the United States, though net prices after insurance and rebates vary widely. Retatrutide pricing will depend on its approved indications, competitive positioning, insurance coverage decisions, and Eli Lilly's pricing strategy.

Insurance coverage decisions cannot be predicted at this time. Coverage for obesity medications has historically been limited, though it has been expanding in recent years as the medical community increasingly recognizes obesity as a chronic disease requiring pharmacological treatment. Coverage will depend on the approved indications, individual insurance plans, and payer policies at the time of potential approval.

If retatrutide receives FDA approval, Eli Lilly would be expected to pursue regulatory submissions in other major markets, including the European Union (through the EMA), Japan, Canada, Australia, and other countries. International approval timelines typically lag behind U.S. approval by 6-18 months, depending on the regulatory agency and submission timing.