"Realistic perceptional experiences in the absence of any external stimuli," is how Liliendeld defines hallucinations. Brain scans during visual hallucinations have reported that the visual cortex becomes active, just as it would if it were actually processing visual stimuli. During a hallucinogenic drug-induced trip, pupils become extremely dilated, as seen above.
There are several causes of hallucinations; for example, lack of sleep and mental illness have been linked to hallucinations. According to Wolfe and Pruitt, after four days of severe sleep deprivation may cause hallucinations. Drug use, such as acid or psilocybin (found in "magic" mushrooms) also causes users to hallucinate. In some cultures, hallucinations are viewed as gifts or religious/spiritual communication. Many who believe this try to induce such "trips" through prayer, fasting, or drug usage.
CNN aired a video of a 50's housewife who voluntarily took Lysergic Acid Diethylamide (acid) under a controlled study. The hospital where the experiment was held was considering hallucinogenic drugs as a form of psychotherapy. The experimenters thought that the psychedelic experience would help the users to become more spiritual, to come to a better understanding of themselves, and possibly treat those suffering from depression or addiction.
When the video above was filmed, the hallucinogenic drugs used were legal. Now that they have been made illegal, the effects of taking such drugs would presumably be more harmful than helpful. In some cases, flashbacks occur, which can persist and cause significant distress or impairment in social or occupational situations. A condition called hallucinogen-induced persisting perceptual disorder, also known as "perma-tripping" can also occur. Many individuals who suffer from HPPD cannot maintain normal relationships or careers and do not function well in society.
Because of the possible negative effects, inducing hallucinations through the use of drugs in individuals that are struggling to come to terms with themselves or that suffer from addiction/depression would likely be a poor choice. Looking at HPPD and how it makes fulfilling a "normal" life extremely difficult, it would more likely have negative effects on individuals who are seeking help rather than benefit them. Hallucinations involved with drugs had the potential to be a very dangerous form of psychotherapy, but the risks had been noted and the drugs were made illegal.