Substances that cause hallucination—perception of things or feelings that have no foundation in reality—when ingested.

Hallucinogens, or psychedelics, are substances that alter users' thought processes or moods to the extent that they perceive objects or experience sensations that in fact have no basis in reality. Many natural and some synthetic substances have the ability to bring about hallucinations. In fact, because of the ready market for such chemicals, they are manufactured in illegal chemical laboratories for sale as hallucinogens. LSD (lysergic acid diethylamide) and many so-called designer drugs have no useful clinical function.

Hallucinogens have long been a component in the religious rites of various cultures, both in the New and Old Worlds. Among the oldest are substances from mushrooms or cactus that have been in use in Native American rites since before recorded history. Hallucinogenic mushrooms have been used for centuries in rites of medicine...