© 2015 American Heart Association, Inc. Patients with atrial fibrillation (AF) often take multiple medications. Methods and Results-We examined characteristics and compared adjusted outcomes between rivaroxaban and warfarin according to number of concomitant baseline medications and the presence of combined cytochrome P450 3A4 and P-glycoprotein inhibitors in the Rivaroxaban Once Daily Oral Direct Factor Xa Inhibition Compared With Vitamin K Antagonism for Prevention of Stroke and Embolism Trial in Atrial Fibrillation (ROCKET AF) study. At baseline, 5101 patients (36%) were on 0 to 4 medications, 7298 (51%) were on 5 to 9, and 1865 (13%) were on ≥10. Although polypharmacy was not associated with higher risk of stroke or non-central nervous system embolism (adjusted hazard ratio, 1.02 for ≥10 versus 0-4 medications; 95% confidence interval, 0.76-1.38), it was associated with higher risks of the combined end point of stroke, non-central nervous system embolism, vascular death, or myocardial infarction (adjusted hazard ratio, 1.41 for ≥10 versus 0-4 medications; 95% confidence interval, 1.18-1.68) and nonmajor clinically relevant or major bleeding (adjusted hazard ratio, 1.47 for ≥10 versus 0-4 medications; 95% confidence interval, 1.31-1.65). There was no significant difference in primary efficacy (adjusted interaction P=0.99) or safety outcomes (adjusted interaction P=0.87) between treatment groups by number of medications. Patients treated with 0 to 4 medications had lower rates of major bleeding with rivaroxaban (adjusted hazard ratio, 0.71; 95% confidence interval, 0.52-0.95; interaction P=0.0074). There was no evidence of differential outcomes in those treated with ≥1 combined cytochrome P450 3A4 and P-glycoprotein inhibitors. Conclusions-In a population of patients with atrial fibrillation, two thirds were on ≥5 medications. Increasing medication use was associated with higher risk of bleeding but not stroke. Rivaroxaban was tolerated across complex patients on multiple medications.