Hallucinogens are a class of drugs that cause profound distortions in a person’s perceptions of reality. These substances, often man-made, are also found in certain plants and mushrooms. Hallucinogens include LSD and PCP. According to the National Institute on Drug Abuse, large doses of marijuana can cause hallucinations, delusions and a loss of the sense of personal identity. These effects are generally temporary. However, the drug can also exacerbate symptoms of schizophrenia in people with this mental illness. Cannabis can also cause symptoms similar to those of other hallucinogens.