OBJECTIVE: Metformin was approved by the Food and Drug Administration in 1995 subject to the conduct of a randomized trial to evaluate the risk of lactic acidosis or other serious adverse events (SAEs) with this agent, under usual care conditions. RESEARCH DESIGN AND METHODS: The Comparative Outcomes Study of Metformin Intervention versus Conventional (COSMIC) Approach Study was a randomized, open-label, active-comparator, parallel-group, 1-year trial in type 2 diabetic patients suboptimally controlled on diet or sulfonylurea. Patients received metformin (n = 7,227) or other usual care treatments (n = 1,505). The primary end point was the incidence of SAEs, death, and hospitalization. RESULTS: SAEs occurred in 10.3% (95% CI 9.6-11.1%) of the metformin group and in 11.0% (9.5-12.7%) of the usual care group (P = 0.431). Lactic acidosis did not occur. All-cause mortality (1.1% [0.9-1.4%] vs. 1.3% [0.8-2.0%], P = 0.596) and hospitalization (9.4% [8.8-10.1%] vs. 10.4% [8.9-12.1%], P = 0.229) were similar between groups. CONCLUSIONS: The incidence of SAEs was similar between groups. Lactic acidosis was not observed. Metformin may be safely prescribed for type 2 diabetes if contraindications and warnings are respected. This study demonstrates the utility of large, simple trials for risk evaluation of treatments for common diseases.