What are hallucinogens?

Alter Consciousness: Hallucinogens are a general group of pharmacological agents that can be divided into three broad categories: psychedelics, dissociatives, and deliriants. These classes of psychoactive drugs have in common that they can cause subjective changes in perception, thought, emotion and consciousness. Unlike other psychoactive drugs, such as stimulants and opioids, these drugs do not merely amplify familiar states of mind, but rather induce experiences that are qualitatively different from those of ordinary consciousness. These experiences are often compared to non-ordinary forms of consciousness such as trance, meditation, dreams, or insanity. L. E. Hollister's criteria for establishing that a drug is hallucinogenic are as follows:[1] in proportion to other effects, changes in thought, perception, and mood should predominate; intellectual or memory impairment should be minimal; stupor, narcosis, or excessive stimulation should not be an integral effect; autonomic nervous system side effects should be minimal; and addictive craving should be absent.