Hallucinations are abnormal sensory perceptions that occur while a person is awake and conscious and are unrelated to outside events; in other words, seeing or hearing things that aren't there.

Common hallucinations include:

• Hearing voices when no one has spoken
• Seeing patterns, lights, beings or objects that aren't there
• Feeling a crawling sensation on the skin

Hallucinations related to smell or taste are rare.

Many recreational drugs, including psychedelic drugs such as LSD and certain potent types of marijuana, may cause hallucinations. Hallucinations related to these drugs tend to be visual and may include geometric patterns or haloes around lights. People who have such visual hallucinations after taking drugs usually recognize that their perception is distorted.

Auditory hallucinations are more common in psychotic conditions such as schizophrenia, although they may sometimes be associated with high doses of cocaine, amphetamine, or other stimulant. High doses of stimulant drugs frequently cause a sensation of bugs crawling on or immediately under the skin.

In some cases, hallucinations, may be normal. For example, hearing the voice of, or briefly seeing, a loved one who has recently died can be a part of the grieving process.

There are numerous medical and psychiatric causes of hallucinations. Common causes include:

• Fever, particularly in children and the elderly
• Intoxication or withdrawal from such drugs as marijuana, LSD, cocaine or crack, heroin, and alcohol
• Delirium or dementia
• Sensory deprivation, such as blindness or deafness
• Severe medical illness, including liver failure, kidney failure, and brain cancer
• Some psychiatric disorders, particularly schizophrenia, psychotic depression, and post-traumatic stress disorder

If someone begins to hallucinate and is detached from reality, a prompt medical evaluation should be sought, since many medical conditions that can cause hallucinations may quickly become emergencies. A person who is hallucinating may become agitated, paranoid, and frightened, and should not be left alone.

Call your health care provider, go to the emergency room, or call the local emergency number (such as 911) if someone appears to be hallucinating and is unable to distinguish hallucinations from reality.

The health care provider will perform a physical examination and will obtain a medical history. Blood may be drawn for testing.

Medical history questions may include the following:

• Does the person hear a voice?
• Does the person see something?
• Does the person have a sensation of feeling something or being touched?
• How long have hallucinations been present?
• When did hallucinations first appear?
• Do hallucinations occur just before or after sleep?
• Has there been a recent death or other emotional event?
• What medications is the person taken?
• Does the person use alcohol regularly?
• Does the person use illicit or illegal drugs?
• Are the hallucinations related to a traumatic event?
• Is there agitation?
• Is there confusion?
• Is there a fever?
• Is there a headache?
• Is there vomiting?