A hallucinogen is a drug or chemical capable of producing hallucinations. A hallucination is a false perception through one of the senses (for example, seeing or hearing something that is not there).
Hallucinogens can be produced naturally or synthetically. The most commonly known hallucinogen is synthetic lysergic acid diethylamide (LSD) which is sold as a liquid, an absorbent tab or small square of paper. Natural hallucinogenic chemicals are found in plants such as the peyote cactus (mescaline) and some mushrooms (psilocybin).
Certain drugs such as cannabis and ecstasy may produce hallucinogenic effects at high doses or in other circumstances.