Retatrutide is an investigational once-weekly injectable from Eli Lilly that activates three gut hormone receptors — GLP-1, GIP, and glucagon — and produced roughly 24 percent body-weight reduction in a 48-week Phase 2 trial. It is not FDA approved as of 2026. Phase 3 TRIUMPH trials are ongoing, with submission expected around 2026 and possible approval in 2027.
What Retatrutide Is
Retatrutide (development code LY3437943) is a synthetic peptide engineered to act as a single molecule on three receptors that influence appetite, glucose, and energy expenditure:
- GLP-1 receptor — slows gastric emptying, reduces appetite, enhances insulin response to meals
- GIP receptor — adds an insulinotropic effect and is thought to improve adipose tissue function
- Glucagon receptor — increases energy expenditure, promotes hepatic fat oxidation, and stimulates lipolysis
It is administered as a once-weekly subcutaneous injection, with the dose escalated gradually over several months to limit gastrointestinal side effects.
Mechanism: Why a Triple Agonist Matters
Existing GLP-1 medications work mainly by reducing energy intake (appetite suppression, slower gastric emptying). Adding glucagon receptor activity introduces an “energy out” lever — glucagon raises basal metabolic rate and promotes use of stored fat. The challenge has historically been balancing glucagon (which can raise blood glucose) with strong GLP-1 activity (which lowers it). Retatrutide is calibrated so the GLP-1/GIP glucose-lowering effects dominate while the glucagon-driven energy expenditure adds weight loss.
Phase 2 Trial Results
The pivotal Phase 2 obesity trial (Jastreboff et al., NEJM 2023) enrolled 338 adults with obesity without diabetes and randomized them to retatrutide 1, 4, 8, or 12 mg weekly or placebo for 48 weeks.
| Dose | Mean weight loss at 48 weeks | Achieving ≥15% loss |
|---|---|---|
| Placebo | ~2.1% | ~2% |
| 1 mg | ~8.7% | ~26% |
| 4 mg | ~17.1% | ~58% |
| 8 mg | ~22.8% | ~74% |
| 12 mg | ~24.2% | ~83% |
At the highest dose, the weight-loss curve had not flattened by week 48 — suggesting more loss could occur with longer treatment. A separate Phase 2 trial in type 2 diabetes showed A1C reductions of approximately 1.6 to 2.2 percent and weight loss of 9 to 17 percent at 36 weeks.
How Retatrutide Compares to Other Weight-Loss Drugs
| Drug | Receptors | Approx. weight loss | Approval status (2026) |
|---|---|---|---|
| Semaglutide (Wegovy) | GLP-1 | ~15% at 68 weeks | FDA approved |
| Tirzepatide (Zepbound) | GLP-1 + GIP | ~21% at 72 weeks | FDA approved |
| Retatrutide | GLP-1 + GIP + glucagon | ~24% at 48 weeks (Phase 2) | Investigational |
| CagriSema | GLP-1 + amylin | ~15-23% (varying readouts) | Investigational |
Cross-trial comparisons should be interpreted with care — different study designs, populations, and durations affect headline numbers. Head-to-head trials would be needed for a definitive ranking.
Phase 3 TRIUMPH Program
Eli Lilly’s Phase 3 program for retatrutide includes:
- TRIUMPH-1 — obesity without diabetes
- TRIUMPH-2 — obesity in adults with type 2 diabetes
- TRIUMPH-3 — obesity with cardiovascular disease
- TRIUMPH-4 — long-term weight maintenance and durability
- Separate Phase 3 trials evaluating retatrutide for type 2 diabetes glycemic outcomes
Topline data readouts are anticipated across 2024-2026, with FDA submission expected around 2026.
Expected Side Effects
From Phase 2 data, the side-effect profile resembles other GLP-1-based therapies, with possibly higher gastrointestinal rates at top doses:
- Nausea — common, especially during dose titration
- Vomiting
- Diarrhea
- Constipation
- Decreased appetite
- Mild heart-rate elevation (a few beats per minute) at higher doses
- Injection site reactions
- Gallbladder-related events (theoretical class concern)
- Pancreatitis (rare, class concern monitored in trials)
Slow dose escalation reduced the proportion of participants who discontinued for GI reasons in Phase 2. The full safety picture will become clearer with Phase 3 data in larger populations.
Who Might Eventually Be a Candidate
If approved on indications similar to existing GLP-1 weight-loss drugs, retatrutide would likely be considered for:
- Adults with a body mass index of 30 or higher (obesity)
- Adults with a BMI of 27 or higher with at least one weight-related condition (hypertension, type 2 diabetes, dyslipidemia, obstructive sleep apnea)
- Possibly adults with type 2 diabetes, pending Phase 3 glycemic results
Cardiovascular disease, hepatic steatosis, and obesity with diabetes are likely focal indications. Approval and labeling will define exact populations.
Cost and Access Expectations
Pricing is not yet set. By analogy, branded GLP-1 obesity drugs in 2026 list around $1,000 to $1,400 per month before insurance and rebates. Retatrutide is expected to be priced in a similar range at launch. Insurance coverage for weight-loss drugs remains inconsistent in 2026, though several large payers and employer plans expanded coverage in 2025. Compounded retatrutide is not legally available — only branded clinical trial supply exists.
Open Questions
- Whether Phase 3 weight loss will match the ~24% Phase 2 signal in broader populations
- Long-term durability after the first year
- Whether the glucagon component affects glycemia or lipid markers unfavorably in some subgroups
- Cardiovascular outcomes — a dedicated CV outcomes trial is part of the program
- Effects in metabolic dysfunction-associated steatohepatitis (MASH), where the glucagon component may be especially relevant
How It Fits the Broader Treatment Landscape
Retatrutide is one of several next-generation incretin therapies in late-stage development. Others — including orforglipron, CagriSema, and survodutide — target different mechanisms. For context on standard options and goals, see our overview of treatment and A1C levels.
What to Discuss with a Clinician
- Whether weight loss is a treatment goal and what targets are reasonable
- Whether existing approved options (semaglutide, tirzepatide, liraglutide, naltrexone-bupropion, phentermine-topiramate) have been tried
- Whether clinical trial participation in retatrutide or other emerging therapies is an option
- Cardiovascular, kidney, and gastrointestinal history that could influence drug choice
- Diet, activity, sleep, and behavioral support — incretin drugs work alongside, not instead of, these
The Bottom Line
Retatrutide is an investigational triple-receptor agonist (GLP-1, GIP, glucagon) from Eli Lilly that produced about 24 percent weight loss in Phase 2 — the highest figure reported for any incretin-based agent so far. It is not FDA approved as of 2026. Phase 3 TRIUMPH trials are ongoing, with submission anticipated around 2026 and possible approval in 2027. Side effects in early trials were predominantly gastrointestinal and resembled the class, with slow titration easing tolerability. The drug is unavailable outside clinical trials; talk to your clinician about approved alternatives or clinical trial participation if weight loss is a major treatment goal.
