Hallucinogens are a diverse group of drugs that alter perception. They alter awareness of surrounding objects and conditions. They can also alter thoughts and feelings. They cause hallucinations, or sensations and images that seem real though they are not. Hallucinogens can be found in some plants and mushrooms (or their extracts) or can be human-made. People have used hallucinogens for centuries, mostly for religious rituals. Common hallucinogens include the following: ayahuasca, DMT, LSD, peyote, and psilocybin.