The fourth edition of The Diagnostic and Statistical Manual of Mental Disorders defines delirium as "a reversible state of confusion with a reduced level of consciousness manifest as an inability to focus, sustain or shift attention." Pharmacologic agents are important contributors to delirium in hospitalized elderly patients and those patients in the perioperative state. Delirium is especially problematic in patients who are given multiple agents, including hypnotics and analgesics. With an appropriate history, physical examination, and investigations, the causes of delirium can be categorized and are usually due to drug intoxication, metabolic disorders, infection, cardiac disorders, stroke, trauma, reactions to pain, or isolation.