Three double-blind, placebo-controlled, three-parallel-group, multicenter phase 3 trials were conducted to assess the efficacy and safety of CP-945,598 for weight loss and weight-loss maintenance. Two trials were designed to be 2 years in duration (in obese and overweight patients) and one as a 1-year study (in obese and overweight patients with type 2 diabetes). However, the 2-year trials and the CP-945,598 development program were terminated before completion due to changing regulatory perspectives of CB1 receptor-related drugs. In total, 1,253 and 2,536 participants in the two 2-year multinational and North American studies were randomized to 10-mg CP-945,598 (n = 360; 718); 20-mg CP-945,598 (n = 534, 1,084) and placebo (n = 359, 734), respectively; and 975 participants were randomized to 10-mg CP-945,598 (n = 318); 20-mg CP-945,598 (n = 320); and placebo (n = 337) in the 1-year multinational diabetes trial. Baseline demographics were similar between treatment groups within each trial. One year of treatment with CP-945,598 resulted in a dose-related mean percentage reduction from baseline body-weight in all trials. A significant proportion of all participants also achieved 5% and 10% weight loss after 1 year. In participants with mainly well-controlled type 2 diabetes, the combination of lifestyle and CP-945,598 induced substantial improvements in glycemic control. The most frequent adverse events (AEs) for CP-945,598 were: diarrhea, nausea, nasopharyngitis, and headache. Self-reported experiences of anxiety and suicidal thoughts were higher with CP-945,598 than placebo, as were the incidence of depression and depressed mood. However, the reported increases in psychiatric symptoms were not consistently dose dependent.