Hallucinogens are among the oldest known group of drugs used for their ability to alter human perception and mood. For centuries, many of the naturally occurring hallucinogens found in plants and fungi have been used for a variety of shamanistic practices. In more recent years, a number of synthetic hallucinogens have been produced, some of which are much more potent than their naturally occurring counterparts. The biochemical, pharmacological, and physiological basis for hallucinogenic activity is not well understood. Even the name for this class of drugs is not ideal, since hallucinogens do not always produce hallucinations.
However, taken in non-toxic dosages, these substances produce changes in perception, thought, and mood. Physiological effects include elevated heart rate, increased blood pressure, and dilated pupils. Sensory effects include perceptual distortions that vary with dose, setting, and mood. Psychic effects include disorders of thought associated with time and space. Time may appear to stand still and forms and colors seem to change and take on new significance. This experience may be either pleasurable or extremely frightening. It needs to be stressed that the effects of hallucinogens are unpredictable each time they are used.
Weeks or even months after some hallucinogens have been taken, the user may experience flashbacks--fragmentary recurrences of certain aspects of the drug experience in the absence of actually taking the drug. The occurrence of a flashback is unpredictable, but is more likely to occur during times of stress and seem to occur more frequently in younger individuals. With time, these episodes diminish and become less intense.