On a summer morning in 2013, Octavian Mihai entered a softly lit room. He swallowed a capsule of psilocybin, an ingredient found in hallucinogenic mushrooms. Then he put on an eye mask and headphones and lay down on a couch. Mr. Mihai, who had just finished treatment for Stage 3 Hodgkin's lymphoma, was participating in a study looking at whether the drug can reduce anxiety and depression in cancer patients. Throughout that eight-hour session, a psychiatrist and a social worker ... stayed by his side. The results from that study, and a similar small, controlled trial, were striking. About 80 percent of cancer patients showed clinically significant reductions in both psychological disorders, a response sustained some seven months after the single dose. Side effects were minimal. In both trials, the intensity of the mystical experience described by patients correlated with the degree to which their depression and anxiety decreased. Although cancer patients will not have access to therapeutically administered psilocybin anytime soon, the findings add vigor to applications to expand research in a multicenter trial with hundreds of participants. Psilocybin trials are underway in the United States and Europe for alcoholism, tobacco addiction and treatment-resistant depression. Other hallucinogens are also being studied for clinical application. This week, the Food and Drug Administration approved a large-scale trial investigating MDMA, the illegal party drug better known as Ecstasy, for post-traumatic stress disorder.