Delirium is acute confusion caused by a medical problem, or by drug intoxication or withdrawal. It includes problems with attention and memory. People with delirium are not able to focus and may not be able to communicate clearly. They may become paranoid, frightened, or see or hear things that aren't there. They may be awake at night and asleep during the day. People with delirium may be agitated, or they may be sleepy and hard to wake up. They may get out of bed when they should not, which can lead to falls, or pull out intravenous lines or catheters. Delirium is a sign of a medical problem, so a person with delirium needs to be evaluated to figure out the cause.

Delirium is common in people with dementia or other cognitive impairment. However, delirium is not a permanent condition and symptoms usually improve after the medical problem improves. Delirium is different from dementia in that delirium usually comes on quickly and the severity may lessen or worsen throughout the course of a day.

Delirium is associated with many negative outcomes, including increased mortality, length of stay in the hospital, and requirements for additional care, such as nursing home placement. It can make it more difficult to care for a person. It predicts future cognitive problems, though it is not clear whether delirium causes or is a sign of those problems.