A hallucinogen is a drug..."capable of inducing illusions, hallucinations, delusions, paranoid ideations, and other alterations of mood and thinking. Despite the name, the feature that distinguishes these agents from other classes of drugs is their capacity to induce states of altered perception, thought, and feeling that are not experienced otherwise."
Hallucinogens may be ingested accidentally, as when lysergic acid diethyamide was first synthesized. They have roles in traditional medicine, especially shamanism, and in experimental Western psychotherapy, where they may provide insights, or actual psychosomatic change.
There was some research into the use of hallucinogens as chemical weapons, but their unpredictability was such that even though they might be disabling and nonlethal, they also could produce effects that would be unwise to produce in heavily armed military forces.