Hallucinations are defined as a perception of and external object when no object is really present. There are many reasons why people have hallucinations. Some reasons for hallucinations are prescribed drugs (SSRI), illegal drugs (LSD), and sleeping disorders (Narcolepsy).
Some causes of hallucinations are from taking drugs, both prescribed and illegal. Both produce the same effect. The hallucinations from these have been described as déjà vu or hearing or seeing thing that aren't really there.
Prescribed drugs such as SSRI which is used for ulcers has been reported to have hallucinations as a side effect. Dronabinol, which is used for the treatment of nausea in chemotherapy, has had the same reports. People with Parkinson's disease have complained of hallucinations when taking an antibiotic called selegiline.
Illegal drugs have also been reported to produce hallucinations. These drugs are called hallucinogens. Lysergic Acid Diethylamide (LSD) is a commonly used hallucinogen among the under ground drug world. Hallucinations from this are very unrealistic. Users say that they see very vivid color and geometric shapes when on a "trip". They tend to hear and see things that aren't there. For example, they will hear someone call their name or see something preposterous like a purple elephant.
Peyote is another illegal drug that creates hallucinations for the user. It is much like LSD in that it has the same hallucinations. It is made from a small cactus in the southwestern region of the U.S. and in parts of Mexico.
There have been several studies made on hallucinations and the effects of drugs in relation to hallucinations. Ronald Seigle did a study on the effects of peyote on the Huichol Indians. He concluded that the hallucinations of the Indians were exceptionally similar to that of college kids doing similar drugs. He believed that the portions of the brain that respond to incoming stimuli become disorganized...