How each drug works
Semaglutide is a GLP-1 receptor agonist. It mimics the incretin hormone glucagon-like peptide-1, stimulating insulin secretion in a glucose-dependent manner, slowing gastric emptying, and reducing appetite via central GLP-1 receptors in the hypothalamus and brainstem. Its half-life is approximately 160 hours, achieved through fatty acid conjugation and albumin binding that make once-weekly subcutaneous dosing feasible.
Retatrutide (LY3437943) agonizes three receptors: GLP-1R, the glucose-dependent insulinotropic polypeptide receptor (GIPR), and the glucagon receptor (GCGR). Compared to endogenous hormones, retatrutide shows approximately 8.9-fold greater potency at GIPR and modestly reduced potency at both GCGR and GLP-1R. In preclinical models, glucagon receptor activity increased energy expenditure and promoted hepatic fatty acid oxidation. The GIP component appears to amplify appetite suppression beyond what GLP-1 alone produces.
Semaglutide's weight-loss effect is driven primarily by reduced calorie intake. Retatrutide adds an energy expenditure component that semaglutide lacks. That mechanistic difference is the main explanation offered for the larger weight losses in retatrutide Phase 2 data, and it will be tested at Phase 3 scale in the TRIUMPH program.
Retatrutide Phase 2 weight loss data
The main retatrutide obesity dataset comes from a Phase 2, double-blind, placebo-controlled, dose-ranging trial published in the New England Journal of Medicine by Jastreboff et al. 2023, n=338. Participants were adults with obesity or overweight without type 2 diabetes, randomized to once-weekly subcutaneous retatrutide at 1, 4, 8, or 12 mg, or placebo, over 48 weeks.
Results were dose-dependent. The 12 mg group reached a mean 17.5% reduction in body weight at 24 weeks and 24.2% at 48 weeks. The 8 mg group reached 22.8% at 48 weeks. The placebo arm lost approximately 2.1% over the same period. A parallel Phase 2 trial in adults with type 2 diabetes, published in The Lancet (Eli Lilly et al. 2023, Lancet Phase 2 T2D), showed dose-dependent weight reductions of up to 16.9% at 24 weeks with 12 mg, alongside meaningful reductions in HbA1c.
Phase 2 trials establish dose ranges and screen for safety signals. A 338-participant study is not powered to serve as a definitive efficacy estimate, and the placebo arm in a small Phase 2 trial carries more statistical uncertainty than a large Phase 3 comparator arm.
Semaglutide Phase 3 results: the STEP-1 trial
The STEP-1 trial is the core semaglutide weight-loss dataset. Published by Wilding et al. 2021 in the New England Journal of Medicine, n=1,961, it randomized adults with obesity or overweight (with at least one comorbidity, but without type 2 diabetes) 2:1 to semaglutide 2.4 mg once weekly or placebo with lifestyle intervention over 68 weeks.
The semaglutide arm achieved a mean weight loss of 14.9% from baseline, vs 2.4% for placebo. 86% of participants on semaglutide lost at least 5% of body weight, and 69% lost at least 10%. These results supported FDA approval of semaglutide 2.4 mg (Wegovy) for weight management in adults.
The full STEP program enrolled roughly 4,500 adults across multiple Phase 3 trials, covering populations with type 2 diabetes, adolescents, and cardiovascular risk groups. SELECT, a cardiovascular outcomes trial comparing semaglutide 2.4 mg to placebo in adults with pre-existing cardiovascular disease and overweight or obesity, demonstrated significant reduction in major adverse cardiovascular events.
Comparing retatrutide vs semaglutide across trial phases
The 24.2% vs 14.9% figures appear frequently in discussions of these two compounds. Placed alongside each other, they come from different trial phases, different sample sizes, different treatment durations, and different placebo arms. Retatrutide's figure is from a 48-week Phase 2 trial in 338 participants; semaglutide's is from a 68-week Phase 3 trial in 1,961 participants. A shorter treatment period in Phase 2 may underestimate retatrutide's maximum effect if the weight loss curve had not plateaued.
A formal head-to-head comparison at Phase 3 scale is registered as TRANSCEND-T2D-2 (NCT06260722). That Eli Lilly-sponsored, open-label Phase 3 study randomizes adults with type 2 diabetes on stable metformin (with or without an SGLT2 inhibitor) to once-weekly retatrutide vs once-weekly semaglutide, with HbA1c and body weight as co-primary endpoints. Results from that trial will be the first regulatory-grade direct comparison of the two compounds.
The table below summarizes published data from each compound's key trial:
| Feature | Retatrutide 12 mg (Phase 2) | Semaglutide 2.4 mg (Phase 3) |
|---|---|---|
| Key trial | Jastreboff et al. 2023, NEJM | Wilding et al. 2021, NEJM (STEP-1) |
| Sample size | n=338 (multi-dose arms) | n=1,961 |
| Duration | 48 weeks | 68 weeks |
| Mean weight loss | 24.2% | 14.9% |
| Receptor targets | GLP-1R, GIPR, GCGR | GLP-1R |
| Dosing | Once weekly, subcutaneous | Once weekly, subcutaneous |
| Regulatory status | No approval; Phase 3 ongoing | FDA-approved (Wegovy) |
Liver fat and secondary metabolic endpoints
Retatrutide's glucagon receptor component produced a secondary finding that semaglutide has not matched at the same magnitude. In a Phase 2a trial for metabolic dysfunction-associated steatotic liver disease (MASLD), retatrutide at 12 mg produced a mean relative liver fat reduction of 82.4% from baseline at 24 weeks (PMC11271400, Phase 2a MASLD trial), compared to a 0.3% increase in the placebo group. Lower doses showed graded reductions: 57.0% at 4 mg and 42.9% at 1 mg.
Semaglutide has shown liver fat reductions in dedicated studies, but the magnitude is generally smaller. The glucagon receptor's role in driving hepatic fatty acid oxidation is the main mechanistic explanation for the gap. Both compounds reduced blood pressure, triglycerides, and other cardiovascular risk markers in their respective trial programs, though these are secondary observations rather than primary endpoints.
The TRIUMPH-Outcomes trial (NCT06383390, retatrutide Phase 3) is designed to test whether retatrutide's metabolic improvements translate to hard cardiovascular outcomes, with a primary endpoint of major adverse cardiovascular events and kidney function decline in adults with obesity and atherosclerotic cardiovascular disease or chronic kidney disease.
Safety and tolerability
Both compounds show the same dominant adverse event pattern: gastrointestinal events, principally nausea, vomiting, and diarrhea. In the retatrutide Phase 2 NEJM trial, these events were most frequent at 12 mg and concentrated during the dose-escalation period. Most were mild to moderate in severity and did not lead to trial discontinuation at rates meaningfully different from placebo.
Semaglutide's STEP-1 trial reported nausea in 44% of the active arm vs 16% on placebo. Serious adverse event rates were similar across treatment and placebo groups. The shared GLP-1 mechanism explains the overlap: both compounds slow gastric emptying, which produces the gastrointestinal profile as an on-target pharmacological effect.
Retatrutide's Phase 3 TRIUMPH program will generate a substantially larger safety dataset. Until those results are published, the full adverse event profile at scale remains incompletely characterized relative to semaglutide's well-established record.
State of the evidence as of mid-2026
Semaglutide has the larger body of evidence by a wide margin: multiple Phase 3 trials across the STEP program, a cardiovascular outcomes trial, regulatory approval in several jurisdictions, and post-market safety data from real-world prescription use. For researchers studying GLP-1 receptor agonism, semaglutide's dataset is the more mature reference point.
Retatrutide is at an earlier regulatory stage. The Phase 2 weight loss data is the largest single-agent figure reported for a GLP-1-class compound in published literature, and the triple-receptor mechanism provides a plausible explanation for it. Phase 3 TRIUMPH data will determine whether those results hold at scale and across more diverse populations.
For preclinical and in-vitro research using either compound, the handling and reconstitution requirements are broadly similar: lyophilized peptide reconstituted in bacteriostatic water, stored refrigerated after reconstitution. The retatrutide compound overview covers the full mechanism and Phase 2 dataset in more detail. Researchers can use the dosing calculator to work through volume math for either compound based on vial concentration and protocol design.