Older adults, particularly those taking more than one medication, are at increased risk of adverse drug events (ADE). In this study of 6,545 older adult patients who were prescribed at least 3 medications, multidrug interactions (MDI) were identified in 1.3% of medication lists. Psychotropic medications were the most commonly involved medication class; the most common serious ADE were serotonin syndrome, seizures, prolonged QT interval, and bleeding.