ContactRetatrutide: the triple-agonist GLP-1 that hit 28.7% weight loss
Eli Lilly's retatrutide activates GLP-1, GIP, and glucagon receptors simultaneously. Phase 3 TRIUMPH-4 showed 28.7% weight loss at 68 weeks — the highest ever recorded in an obesity trial. Here is what we know about the mechanism, trial data, side effects, and timeline.
By GLP-1 Scout Editorial Team · Published April 5, 2026
Retatrutide (LY3437943) is an investigational weight-loss drug from Eli Lilly that activates three receptor pathways simultaneously: GLP-1, GIP, and glucagon. It is the first triple-agonist to reach Phase 3 clinical trials, and its early data has produced the largest weight-loss numbers ever seen in an obesity trial — 28.7% mean body weight loss at 68 weeks in TRIUMPH-4. The drug is not yet FDA-approved, but its Phase 3 program (TRIUMPH) spans 8 trials with over 5,800 participants, and an NDA filing is expected in late 2026 or early 2027.
Mechanism: why three receptors matter
Tirzepatide (Zepbound) activates two receptors — GLP-1 and GIP. Retatrutide adds a third: the glucagon receptor. Each receptor contributes a different physiological effect:
GLP-1 receptor: suppresses appetite via hypothalamic satiety centers, slows gastric emptying, stimulates glucose-dependent insulin secretion. This is the same pathway targeted by semaglutide and all existing GLP-1 RAs.
GIP receptor: improves insulin sensitivity and shifts fat distribution away from visceral depots toward subcutaneous tissue. This is the second pathway in tirzepatide.
Glucagon receptor: increases resting metabolic rate, promotes lipolysis (breakdown of stored fat), and directly mobilizes liver fat. This is unique to retatrutide among advanced obesity candidates.
The glucagon component is what makes retatrutide fundamentally different from tirzepatide. While tirzepatide works primarily by reducing caloric intake through appetite suppression, retatrutide adds an energy-expenditure component — patients burn more calories at rest. The GLP-1 and GIP components counterbalance glucagon's tendency to raise blood glucose, preserving the metabolic-rate boost without hyperglycemia.
Phase 2 results (NEJM, June 2023)
The Phase 2 trial (Jastreboff et al., New England Journal of Medicine, 2023) randomized 338 adults with obesity to retatrutide at doses of 1 mg, 4 mg, 8 mg, or 12 mg weekly versus placebo for 48 weeks. The results were unprecedented:
| Dose | Mean weight loss at 48 weeks | Patients losing ≥15% | Patients losing ≥20% |
|---|---|---|---|
| Placebo | -2.1% | 2% | <1% |
| 1 mg | -8.7% | — | — |
| 4 mg | -17.1% | 60% | 34% |
| 8 mg | -22.8% | 75% | 54% |
| 12 mg | -24.2% | 83% | 63% |
At the 12 mg dose, the weight-loss curve was still descending at week 48 — no plateau had been reached. This suggested that longer treatment periods would produce even greater weight loss, which the Phase 3 data later confirmed.
Phase 3 TRIUMPH-4: 28.7% at 68 weeks
TRIUMPH-4, the first Phase 3 trial to report results (December 2025), enrolled 445 adults with obesity and knee osteoarthritis. Patients were randomized to retatrutide 9 mg, 12 mg, or placebo for 68 weeks:
12 mg dose: -28.7% mean weight loss (-32.3 kg / ~71 lbs). This is the highest weight loss ever reported in a randomized obesity trial.
9 mg dose: -26.4% mean weight loss (-29.1 kg / ~64 lbs).
Placebo: -2.1%.
Knee osteoarthritis pain: 12.0% (12 mg) and 14.1% (9 mg) were completely pain-free at 68 weeks versus 4.2% on placebo.
Cardiovascular markers: 12 mg reduced systolic blood pressure by 14.0 mmHg, with improvements in non-HDL cholesterol, hsCRP, and triglycerides.
Side effects: familiar GI plus a new signal
Retatrutide shares the gastrointestinal side effect profile of other GLP-1-class drugs — nausea (~44-60% at highest doses), diarrhea, vomiting, and constipation. These are predominantly mild-to-moderate and concentrated during dose escalation. However, TRIUMPH-4 revealed a safety signal not seen with semaglutide or tirzepatide:
Dysesthesia: the new side effect
Dysesthesia — described as tingling, burning, or abnormal skin sensitivity — occurred in 20.9% of the 12 mg group and 8.8% of the 9 mg group, versus 0.7% on placebo. Cases were generally classified as mild and rarely led to treatment discontinuation. The mechanism is hypothesized to relate to glucagon receptor expression in nervous tissue. Neither semaglutide nor tirzepatide activates glucagon receptors, which is why this side effect is unique to retatrutide.
Retatrutide also produced dose-dependent heart rate increases of approximately 5-10 bpm, peaking around week 24 and declining thereafter. This is higher than semaglutide (1-4 bpm) or tirzepatide.
Dose escalation and treatment doses
Phase 3 evaluates maintenance doses of 4 mg, 9 mg, and 12 mg, all administered by weekly subcutaneous injection. Escalation starts at 2 mg and increases every 4 weeks over approximately 12 weeks to the target dose. Phase 2 data showed that starting at 2 mg rather than 4 mg reduced GI side effects during escalation.
Timeline: when could retatrutide be available?
Phase 3 TRIUMPH program: 8 trials across obesity, type 2 diabetes, cardiovascular disease, sleep apnea, liver disease, and osteoarthritis. Seven additional readouts expected throughout 2026.
NDA submission: expected late 2026 or Q1 2027, contingent on remaining TRIUMPH data.
FDA approval: projected late 2027 to early 2028 (standard ~10-month review).
No brand name has been assigned yet.