Retatrutide vs. Wegovy: What the Data Actually Shows

The Temptation of Cross-Trial Comparisons

When retatrutide Phase 2 data showed up to 24.2% mean body weight loss at 48 weeks, clinicians and patients immediately asked how it compares with semaglutide 2.4 mg (Wegovy). Answering takes care, because the two drugs were studied in different trials, in different patient populations, at different times.

Cross-trial comparisons (sometimes called naive indirect comparisons) have built-in limits. Differences in baseline BMI, demographics, comorbidity profiles, diet and exercise counseling intensity, and even placebo response rates can all influence outcomes. The only definitive way to compare two therapies is a head-to-head randomized controlled trial, which has not been reported for retatrutide versus semaglutide 2.4 mg in obesity. (TRANSCEND-T2D-2 compared retatrutide with semaglutide 2 mg, the Ozempic diabetes dose, in type 2 diabetes; see below.)

The Numbers at a Glance

In the STEP 1 trial, semaglutide 2.4 mg produced a mean body weight reduction of approximately 14.9% at 68 weeks in adults with obesity or overweight (BMI ≥ 30, or ≥ 27 with at least one weight-related comorbidity) without diabetes.

In the Phase 2 trial of retatrutide, the highest dose group (12 mg) reached a mean weight loss of 24.2% at 48 weeks in a similar population of adults with obesity. That trial was shorter, and weight was still falling when it ended.

Update, TRIUMPH-4 Phase 3 data (December 2025): We now have data over the same length of time. TRIUMPH-4 enrolled 445 participants with obesity and knee osteoarthritis over 68 weeks, the same duration as STEP 1. The 12 mg group reached 28.7% mean weight loss and the 9 mg group 26.4%, compared with 2.1% on placebo. That removes the earlier caveat about mismatched trial lengths and allows a more direct comparison: at the same 68-week timepoint, retatrutide at 12 mg produced roughly twice the weight loss of semaglutide 2.4 mg (28.7% vs. 14.9%).

The headline figures show a large gap, though some important context still applies.

Mechanism: Triple vs. Single Agonist

Semaglutide is a selective GLP-1 receptor agonist. It reduces appetite, slows stomach emptying and acts on brain pathways involved in feeling full.

Retatrutide is a triple agonist that acts on GIP, GLP-1 and glucagon receptors at once. Adding glucagon receptor agonism is thought to raise energy expenditure through effects on liver metabolism and heat production. GIP receptor agonism may improve insulin sensitivity and possibly have synergistic effects on appetite.

This difference in mechanism matters. Activating the glucagon receptor is a fairly new approach in obesity treatment, and how much it adds to weight loss beyond GLP-1 and GIP agonism is still being studied.

Population and Trial Design Differences

Several differences between the STEP program and the retatrutide Phase 2 trial complicate direct comparison:

• Trial duration: The duration mismatch that previously complicated comparison has been partially resolved. TRIUMPH-4 ran for 68 weeks, matching STEP 1. However, TRIUMPH-4 enrolled a specific population (obesity with knee osteoarthritis), not the general obesity population studied in STEP 1.
• Baseline characteristics: Mean starting BMI and body weight differed between trials, which can affect the percentage of weight lost. TRIUMPH-4 participants were required to have knee OA, which may affect their starting characteristics and response.
• Sample size: STEP 1 enrolled over 1,900 participants. TRIUMPH-4 enrolled 445 participants. TRIUMPH-1 (general obesity population, 2,339 participants), reported in 2026, provides a more directly comparable dataset, though over 80 weeks rather than 68.
• Lifestyle intervention: Both trials included lifestyle counseling, but how intensive and standardized it was can vary.

Tolerability Considerations

Both agents share GLP-1 receptor agonism, so gastrointestinal side effects are common to both. TRIUMPH-4 Phase 3 data showed nausea (43.2%), diarrhea (33.1%), and vomiting (20.9%) at the 12 mg dose, with discontinuation rates of 18.2% at 12 mg versus 4.0% on placebo. One notable difference is dysesthesia (abnormal skin sensation), which occurred in 20.9% of the retatrutide 12 mg group and is not linked to semaglutide. It may be related to the glucagon receptor component, and it is a meaningful difference in tolerability. Definitive tolerability comparisons still require head-to-head data.

What We Still Need

With TRIUMPH-4 giving the first Phase 3 data at 68 weeks, the efficacy comparison is more concrete. Retatrutide’s weight loss advantage over semaglutide 2.4 mg looks substantial and now has Phase 3 evidence behind it. TRIUMPH-1, the core obesity trial and the population closest to STEP 1, has since reported 28.3% mean weight loss at 12 mg over 80 weeks (efficacy estimand; published in the New England Journal of Medicine on September 29, 2026). In October 2026, TRANSCEND-T2D-2 gave the first head-to-head data against semaglutide, at the 2 mg diabetes dose in 1,298 adults with type 2 diabetes: over 80 weeks, weight fell 19.5% and 20.9% on retatrutide 9 mg and 12 mg versus 9.4% on semaglutide, and A1C fell 2.2 points versus 1.8. Those are Lilly’s topline figures and have not yet been peer reviewed.

For clinicians and patients, the main point is this: retatrutide’s Phase 3 data confirm a meaningful efficacy advantage, but the evidence for semaglutide 2.4 mg is far more mature, with years of post-marketing data and cardiovascular outcomes evidence (SELECT trial). Treatment decisions should rest on the individual patient, on tolerability (including retatrutide’s distinct dysesthesia signal) and on all the evidence available when the drug is prescribed.