Hallucinogenic drugs are chemicals that grossly distort perception. Some types of these drugs include lysergic acid diethylamide (LSD), phencyclidine (PCP), and mescaline. Hallucinogens resemble a transmitter called serotonin; this similarity permits the hallucinogens to attach to serotonin receptors.
Serotonin (5-HT) has a few types or receptors with differing properties. The hallucinogens attach to a type that contribute to perception, and are very abundant in the brain. This attachment produces abnormalities to the receptors, hence, an abnormality to perception.