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Hallucinogens are natural and human-made substances that often cause people to believe they see random colors, patterns, events, and objects that do not exist. The hallucinatory experiences can either be very pleasant or very disturbing. Many different types of substances are classified as hallucinogens because of their capacity to produce such hallucinations. These substances come in the form of pills, powders, liquids, gases, and plants that can be eaten. In the body, hallucinogens stimulate the nervous system. Effects include the dilation (widening) of the pupils of the eyes, constriction of certain arteries, and rising blood pressure.

Hallucinogens have long been a part of the religious rites of various cultures throughout history. Tribal shamen or medicine men...