Educational reference only. Informational material drawn from peer-reviewed literature. Not medical advice and not a recommendation to use, dose, or modify any therapy. Decisions about hormone therapy should be made with a qualified healthcare provider.
Overview
Overview
Retatrutide (LY3437943) is a synthetic triple hormone receptor agonist, the first compound in clinical development that simultaneously activates three receptors: GIP (glucose-dependent insulinotropic polypeptide), GLP-1 (glucagon-like peptide-1), and glucagon (GCG). This triple-receptor mechanism distinguishes retatrutide from semaglutide (GLP-1 monoagonist) and tirzepatide (dual GIP/GLP-1 agonist), and has produced the largest weight loss results ever recorded in obesity drug trials.
Retatrutide is being developed by Eli Lilly, the same company that brought tirzepatide to market. As of May 2026, the compound is in Phase 3 clinical trials with the first positive Phase 3 readout announced in March 2026 (TRANSCEND-T2D-1 for type 2 diabetes) and Phase 3 weight loss data from TRIUMPH-4 showing up to 28.7% body weight loss at 68 weeks.
Development status:
- Obesity: Phase 3 (TRIUMPH-1, TRIUMPH-2, TRIUMPH-3, TRIUMPH-4)
- Type 2 diabetes: Phase 3 (TRANSCEND-T2D-1, positive readout March 2026)
- Knee osteoarthritis with obesity: Phase 3 positive readout (TRIUMPH-4)
- Obstructive sleep apnea with obesity: Phase 3 (TRIUMPH-OSA)
- Chronic kidney disease: Phase 3 (TRIUMPH-CKD)
- MASH (steatohepatitis): Phase 3 (TRIUMPH-MASH)
- Cardiovascular and renal outcomes: Phase 3 (multiple)
- Chronic low back pain: Phase 3 (TRIUMPH-related)
Forms
Available forms & half-lives
| Form | Approximate half-life |
|---|---|
| Subcutaneous injection (investigational; not yet approved) | ~6 days |
What it does
Main effects
- Largest pharmaceutical weight loss results ever recorded: Phase 2 showed mean −24.2% at 48 weeks on 12 mg PMID: 37366315; Phase 3 TRIUMPH-4 (knee osteoarthritis with obesity) showed up to −28.7% at 68 weeks
- Triple-receptor agonism: first compound in development to activate GIP, GLP-1, AND glucagon receptors; the glucagon component is the novel pharmacological feature
- Dramatic liver fat reduction: up to 82% reduction in MASLD/MASH at 24 weeks in Phase 2 substudy [Sanyal et al., Nature Medicine 2024]; far exceeds effects of GLP-1 or GIP/GLP-1 therapy
- Increased energy expenditure via glucagon agonism: distinguishes from semaglutide and tirzepatide which produce weight loss primarily through appetite reduction
- Net glucose-lowering despite glucagon agonism: predominant insulinotropic effects from GIP and GLP-1 overwhelm glucagon-induced glucose elevation
- Diabetes prevention potential: 72% of Phase 2 participants with prediabetes at baseline reverted to normoglycemia
- Active Phase 3 program across multiple indications: obesity, T2D, knee OA, OSA, CKD, MASH, low back pain
What to watch for
Common side effects
- NOT FDA-approved: investigational; not legally available for prescription outside clinical trials; FDA submission anticipated 2026–2027
- Gastrointestinal effects (nausea, vomiting, diarrhea, constipation): dose-dependent pattern similar to semaglutide and tirzepatide
- Pancreatitis risk: class concern; will likely receive class warning at approval
- Medullary thyroid carcinoma: theoretical risk based on rodent studies of GLP-1 class drugs; will likely receive boxed warning at approval
- Acute kidney injury: usually secondary to dehydration from GI effects
- Quality concerns are particularly significant: no FDA-approved manufacturing standards; research-chemical products may have identity/concentration/purity/sterility issues; counterfeits documented
- Long-term effects of chronic glucagon agonism in humans less well-characterized than GLP-1 effects: potential considerations include effects on hepatic glucose production, energy expenditure regulation, protein metabolism, bone density
- Hypoglycemia in combination with insulin or sulfonylureas (low risk as monotherapy)
- Heart rate increase observed in Phase 2 (modest, similar to other GLP-1 class drugs)
- Lean mass loss: significant weight loss includes both fat and lean mass; possibly less lean mass loss than expected due to glucagon-mediated energy expenditure effects (under investigation)
- Compounded retatrutide is legally questionable: the compound is not in widespread approved use, which is the typical justification for compounding
Key facts
Key facts worth knowing
- Retatrutide is a synthetic peptide conjugated to a fatty diacid moiety, similar in basic design to semaglutide and tirzepatide. The fatty acid modification enables albumin binding and extended half-life for once-weekly subcutaneous dosing.
- The compound activates three receptors rather than one or two. This triple mechanism produces effects beyond what GLP-1 or GIP/GLP-1 agonism alone can achieve, particularly through the glucagon agonism component, which increases energy expenditure and reduces hepatic fat.
- Phase 2 efficacy data was remarkable. In the foundational Phase 2 obesity trial, 48 weeks of retatrutide produced mean weight loss of 24.2% at the 12 mg dose, the largest mean weight loss ever reported in a pharmaceutical obesity trial at that point. PMID: 37366315
- Phase 3 data has continued to demonstrate exceptional efficacy. TRIUMPH-4 (knee osteoarthritis with obesity) data released in 2025–2026 demonstrated up to 28.7% body weight loss at 68 weeks on the 12 mg dose, approximately 71 lbs of weight loss on average.
- The glucagon agonist component is the most novel pharmacological feature. Glucagon agonism: increases energy expenditure (resting metabolic rate), reduces hepatic fat (potentially dramatic effects on liver disease), mobilizes glycogen and fat stores. Historically, glucagon agonism was considered counterproductive in diabetes treatment (since glucagon raises blood sugar). The retatrutide design balances glucagon agonism against the much stronger GIP and GLP-1 effects, producing net glucose-lowering despite the glucagon component.
- The liver fat reduction results have been particularly dramatic. A Phase 2 substudy in MASLD/MASH demonstrated up to 82% reduction in liver fat at 24 weeks, far exceeding effects of GLP-1 monoagonist or GIP/GLP-1 dual therapy. Sanyal AJ et al., Nature Medicine 2024
- The compound is NOT currently approved or legally available for prescription. Despite this, retatrutide has appeared in non-medical contexts through: unregulated sources (sold for "research purposes only" with significant quality concerns), compounded products (legally questionable; not FDA-approved), underground sources (counterfeit, unverified), and compounding pharmacies (limited and contested).
Legal status
Legal status
Investigational; not yet approved. Not legally available for prescription or sale outside clinical trials. Possession and use of research chemical retatrutide for human consumption is in regulatory gray area in most jurisdictions.