Deliriants are a class of hallucinogen. The term was introduced by David F. Duncan and Robert S. Gold to distinguish these drugs from psychedelics, such as LSD, due to their primary effect of causing delirium, as opposed to the more lucid states produced by other hallucinogens (psychedelics and dissociatives).

The delirium produced is characterized by stupor, utter confusion, confabulation, and regression to "phantom" behaviors such as disrobing and plucking. Other commonly reported behaviors include holding full and life like conversations with imagined people, finishing a complex, multi-stage action (such as getting dressed) and then suddenly discovering you had not even begun yet, and being unable to recognize one's own reflection in a mirror.

The effects have been likened to sleepwalking, a fugue state or a psychotic episode (particularly in that the subject has minimal control over their actions and little to no recall of the experience). This is a notable departure from typical hallucinogens.