Hallucinations are sensory perceptions that are unrelated to outside events -- in other words, seeing or hearing things that aren't there.

Hallucinations are abnormal sensory perceptions that occur while a person is awake and conscious. Some 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, such as geometric patterns or haloes around lights. A person who has such visual hallucinations after taking drugs usually recognizes that 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 stimulants. High doses of stimulant drugs also frequently cause a sensation of bugs crawling on or immediately under the skin.

If someone begins to hallucinate and is detached from reality, a prompt medical evaluation should be sought -- 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.

In some cases, however, hallucinations, may be normal. For example, having a hallucination of 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, which can occur with almost any infection, frequently produces hallucinations 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

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 the patient's medical history. Blood may be drawn for testing.

Medical history questions documenting hallucinations in detail may include the following:

• Is there a sensation of a voice (auditory hallucinations)?
• Is there a sensation of something seen (visual hallucination)?
• Is there a sensation of something felt or touched (tactile hallucination)?
• How long have hallucinations been present?
• When did hallucinations first appear (initial onset)?
• Do hallucinations occur just before or after sleep?
• Has there been a recent death or other emotional event?
• What medications are being taken?
• Is alcohol used regularly?
• Are illicit or illegal drugs being used?
• 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?