Which of the following is considered a hallucinogen?

PCP, or phencyclidine, is classified as a hallucinogen because it alters perceptions, thoughts, and feelings, leading to distortions in reality. Users may experience visual and auditory hallucinations, feelings of detachment from the body, and significant changes in mood. This substance interacts with the brain's neurotransmitters and affects multiple systems, creating a unique psychoactive experience that is characteristic of hallucinogens.

The other substances listed do not primarily produce hallucinations or altered perceptions. Caffeine is a stimulant that increases alertness and energy levels without causing hallucinations. Heroin is an opioid, primarily known for its pain-relieving properties and potential for inducing euphoria, but it does not cause hallucinations. Alcohol, while it can impair judgment and coordination, primarily functions as a depressant, not a hallucinogen.