Despite the fact that hallucinogens are one of the oldest known categories of drugs, experts still lack a thorough understanding of the substances that fall in this category. In addition, despite their name, hallucinogenic drugs do not always produce hallucinations.
The effects of these drugs are extremely unpredictable and can differ with each use. Hallucinogens can produce very frightening experiences for users, including changes in perception, thought and mood; illusions and hallucinations; impaired hand-eye coordination; and sensory crossover. Hallucinogens also impair judgment and can lead to numerous other problems often resulting from accidents and poor judgment. In addition, users can experience unpredictable "flashbacks" weeks and even months after using these substances. Examples of hallucinogens include mescaline, lysergic acid diethylamide (LSD), phencyclidine (PCP), and psilocybin and psilocyn, which are chemicals found in certain mushrooms grown in Mexico and Central America.