Hallucinations involve sensing things that aren't there while a person is awake and conscious.

Common hallucinations include:

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

Hallucinations related to smell or taste are rare.

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

Hearing things (auditory hallucinations) is more common in psychotic conditions such as schizophrenia, although it may sometimes occur with high doses of cocaine, amphetamines, or other stimulants. High doses of stimulant drugs can make you feel as though there are bugs crawling on or just 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 many causes of hallucinations, including:

• Being drunk or high, or coming down from such drugs as marijuana, LSD, cocaine or crack, heroin, and alcohol
• Delirium or dementia
• Fever, especially in children and the elderly
• Sensory problem, such as blindness or deafness
• Severe illness, including liver failure, kidney failure, and brain cancer
• Some psychiatric disorders, such as schizophrenia, psychotic depression, and post-traumatic stress disorder

A person who begins to hallucinate and is detached from reality should get checked by a health care professional right away, because many medical conditions that can cause hallucinations may quickly become emergencies. A person who is hallucinating may become nervous, 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 tell hallucinations from reality.

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

Medical history questions may include the following:

• Do you hear a voice?
• Do you see something?
• Do you have a sensation of feeling something or being touched?
• How long have you been having hallucinations?
• When did the hallucinations first appear?
• Do the hallucinations occur just before or after sleep?
• Has there been a recent death or other emotional event?
• What medications are you taking?
• Do you use alcohol regularly?
• Do you use 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?

Addington D, Bouchard RH, Goldberg J, Honer B, Malla A, Norman R, Tempier R. Clinical practice guidelines: treatment of schizophrenia. Can J Psychiatry. 2005;50:7S-57S.

International Early Psychosis Association Writing Group. International clinical practice guidelines for early psychosis. Br J Psychiatry. 2005;48:s120-s124.