Hallucinogens, or psychedelics, are drugs that affect a person's perceptions, sensations, thinking, self-awareness, and emotions.
Some hallucinogens come from natural sources, such as mescaline from the peyote cactus.
Others, such as LSD, are synthetic or manufactured. PCP is sometimes considered an hallucinogen because it has some of the same effects. However, it does not fit easily into any one drug category because it also can relieve pain or act as a stimulant. LSD is manufactured from lysergic acid which is found in ergot, a fungus that grows on rye and other grains. LSD was discovered in 1938 and is one of the most potent mood-changing chemicals.