BACKGROUND Lorcaserin, a selective serotonin 2C receptor agonist that modulates appetite, has proven efficacy for weight management in overweight or obese patients. The cardiovascular safety and efficacy of lorcaserin are undefined. METHODS We randomly assigned 12,000 overweight or obese patients with atherosclerotic cardiovascular disease or multiple cardiovascular risk factors to receive either lorcaserin (10 mg twice daily) or placebo. The primary safety outcome of major cardiovascular events (a composite of cardiovascular death, myocardial infarction, or stroke) was assessed at an interim analysis to exclude a noninferiority boundary of 1.4. If noninferiority was met, the primary cardiovascular efficacy outcome (a composite of major cardiovascular events, heart failure, hospitalization for unstable angina, or coronary revascularization [extended major cardiovascular events]) was assessed for superiority at the end of the trial. RESULTS At 1 year, weight loss of at least 5% had occurred in 1986 of 5135 patients (38.7%) in the lorcaserin group and in 883 of 5083 (17.4%) in the placebo group (odds ratio, 3.01; 95% confidence interval [CI], 2.74 to 3.30; P<0.001). Patients in the lorcaserin group had slightly better values with respect to cardiac risk factors (including blood pressure, heart rate, glycemic control, and lipids) than those in the placebo group. During a median follow-up of 3.3 years, the rate of the primary safety outcome was 2.0% per year in the lorcaserin group and 2.1% per year in the placebo group (hazard ratio, 0.99; 95% CI, 0.85 to 1.14; P<0.001 for noninferiority); the rate of extended major cardiovascular events was 4.1% per year and 4.2% per year, respectively (hazard ratio, 0.97; 95% CI, 0.87 to 1.07; P = 0.55). Adverse events of special interest were uncommon, and the rates were generally similar in the two groups, except for a higher number of patients with serious hypoglycemia in the lorcaserin group (13 vs. 4, P = 0.04). CONCLUSIONS In a high-risk population of overweight or obese patients, lorcaserin facilitated sustained weight loss without a higher rate of major cardiovascular events than that with placebo.