Hallucinations are abnormal sensory perceptions that occur while a person is awake and conscious. Some common hallucinations are hearing voices when no one has spoken, seeing patterns, lights, beings or objects that aren't there, or 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, can 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 his or her 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 because many medical conditions that can cause hallucinations may quickly become emergencies. People who are 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. Some of the common causes include the following:

• 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/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