Hallucinogens are drugs that change one's thought processes, mood, and perceptions. Hallucinogens are the most "consciousness-altering" drugs among the various drug classifications. The most commonly used hallucinogens are LSD, psilocybin mushrooms, peyote, ketamine, and salvia. The use of synthetic hallucinogens such as 25I and 2C is also increasing.

LSD is the most commonly used hallucinogen. Using LSD can lead to feelings of detachment, an altered sense of space and time, and hallucinations. Hallucinations are sensory experiences that are unreal and can be experienced within the various senses: visual, auditory, tactile, etc. LSD can also result in a user experiencing delusions, such as believing that one can fly. Researchers are unsure how LSD specifically works, but recent research suggests that it is a result of the alteration of serotonin receptors in the brain. It is also thought that LSD alters consciousness by loosening or erasing the normal filters between the conscious mind and the outside world.

• Physical Effects: nausea, dizziness, muscle weakness, sweating, dilated pupils
• Psychological Effects: feelings of unreality or depersonalization, disorientation, hallucinations, panic or heightened anxiety response, rapid mood changes
• Behavioral: erratic, unpredictable behavior such as rambling or strange speech

A bad trip is synonymous to having a sustained frightening or uncomfortable experience as a result of using LSD. During a "bad trip," a user may experience intense feelings of paranoia, fear of dying, and intense fear and anxiety. Visual hallucinations could include objects such as spiders, blood, insects, monsters, and skulls (these objects are more commonly reported).

Some hallucinogen users have reported experiencing "flashbacks" or what is more formally known as post-hallucinogen perceptual disorder. Flashbacks are characterized by visual disturbances or other recalled events of the drug experience that emerge long after the drug is out of the body. Hallucinogen use can induce severe anxiety and panic symptoms, as well as psychosis in some users. These symptoms may also persist after "coming down" from the intoxicated phase.