Retatrutide
Triple GIP/GLP-1/glucagon agonist. 28.7% weight loss in Phase 2 trials.
- First synthesized
- 2022
- Primary research area
- Weight / obesity
- FDA status
- Phase 3 (Eli Lilly, 2025–)
- Last updated
- Apr 21, 2026
- Reviewed by
- Peptide Examiner editorial team
What it is
Retatrutide is an investigational triple receptor agonist developed by Eli Lilly, targeting the GLP-1, GIP, and glucagon receptors with a single peptide molecule. The addition of glucagon-receptor activity on top of the GIP/GLP-1 combination that tirzepatide uses is a pharmacological escalation: glucagon agonism is theorized to increase energy expenditure in addition to the appetite-suppressing effects of the other two pathways. Retatrutide is in Phase 3 as of April 2026, with readouts expected to define whether the largest obesity-drug weight-loss efficacy to date holds up under longer durations.
Mechanism of action
Retatrutide is a 39-amino-acid peptide engineered to bind three incretin/metabolic hormone receptors with balanced activity: GLP-1 (insulin secretion, appetite suppression, gastric emptying), GIP (complementary central and peripheral effects), and glucagon (increased hepatic glucose production normally, but in the context of combined receptor activation, net energy expenditure increases). The glucagon-receptor component is counterintuitive — glucagon typically raises blood sugar — but paired with GLP-1 and GIP the net effect favors weight loss via increased thermogenesis and basal metabolic rate. Half-life supports once-weekly dosing.
Research history
The Phase 2 trial (NEJM, August 2023) enrolled 338 adults with obesity across 12 mg and lower-dose arms. The 12 mg group lost a mean 24.2% of body weight at 48 weeks — the largest mean weight loss reported for any obesity drug at the time, higher than tirzepatide's 48-week comparable data. The TRIUMPH Phase 3 trial program (TRIUMPH-1 through TRIUMPH-4, 2024-2026+) is ongoing, evaluating efficacy and safety at 72+ weeks across different populations (obesity, T2D, obstructive sleep apnea, cardiovascular outcomes).
Current trial status
Phase 3 ongoing as of April 2026. TRIUMPH-1 (adults with obesity) and TRIUMPH-2 (adults with T2D and obesity) are the lead efficacy trials; TRIUMPH-3 is evaluating cardiovascular outcomes. Lilly has indicated expectation of FDA approval timing in 2026-2027 assuming positive readouts. Dose-dependent GI tolerability has been consistent with the GLP-1 class.
Regulatory status
Investigational. Not FDA approved. Not available through any legal US pharmacy or compounding pathway. Research-peptide vendors selling substances labeled as 'retatrutide' for research use are subject to FDA enforcement — several received warning letters in December 2024 for doing so. See our FDA timeline. Full regulatory timeline →
Controversies and open questions
The triple-agonist approach is a scientific frontier with real open questions. Does glucagon-receptor activation contribute to higher sustained energy expenditure or does it just modify the PK profile? Early human data suggest the former but longer-duration data will settle it. Also unresolved: whether the larger weight loss comes with proportionally larger muscle mass loss, a sarcopenia concern that scales with efficacy. TRIUMPH trial sub-analyses will eventually quantify this.
Further reading
Frequently asked
When will retatrutide be available?
Eli Lilly has indicated expected FDA approval timing of 2026-2027 depending on Phase 3 TRIUMPH trial readouts. Exact timing depends on regulatory filing and FDA review. It is not yet approved and not available through any legal compounding pathway.
Is retatrutide better than tirzepatide?
On early Phase 2 evidence, yes — the 12 mg dose produced 24.2% mean weight loss at 48 weeks vs tirzepatide's 20.9% at 72 weeks in SURMOUNT-1. Head-to-head trials haven't run. Individual response variability is high and means don't predict individuals.
What's the triple-agonist advantage?
Retatrutide hits three receptors: GLP-1, GIP, and glucagon. The addition of glucagon-receptor agonism is thought to increase energy expenditure via thermogenesis, on top of the appetite-suppression and insulin-effects of GLP-1 and GIP. Whether this explains the larger weight loss, or whether it's a different mechanism, is still debated.
Are research-peptide vendors selling 'retatrutide' legitimate?
No — legitimate sources don't exist yet outside clinical trial enrollment. Vendors selling research-peptide 'retatrutide' have been subjects of FDA warning letters (December 2024). The substance hasn't been FDA approved and isn't legally available for human use.
What's the TRIUMPH program?
Eli Lilly's Phase 3 clinical trial suite for retatrutide. TRIUMPH-1 (adults with obesity), TRIUMPH-2 (T2D with obesity), TRIUMPH-3 (cardiovascular outcomes), and TRIUMPH-4 are the lead trials. Readouts are expected through 2025-2026.