Psychedelic therapy is the use of plants and compounds that can induce hallucinations to treat mental health diagnoses, such as depression and post-traumatic stress disorder (PTSD). Some of the compounds that doctors most frequently use in this form of treatment include psilocybin mushrooms, LSD, and mescaline (peyote). The formal study of psychedelics to treat mental health conditions is relatively new, but emerging research suggests that these psychedelics may help some people with some symptoms, especially when other methods of treatment have failed. Researchers do not know exactly how or why psychedelics work in this manner.