A hallucinogen is a substance that alters perception, mood, and cognitive processes, often leading to visual or auditory experiences that are not based in reality. These substances can be natural, like psilocybin mushrooms and peyote, or synthetic, such as LSD and MDMA. Hallucinogens primarily affect neurotransmitter systems, particularly serotonin, and can induce profound changes in consciousness. Their effects can vary widely depending on the dose, context, and individual differences.