Which of the following substances is not classified as a hallucinogen?

Vivitrol is the correct choice because it is not classified as a hallucinogen. Instead, Vivitrol is a brand name for naltrexone, which is primarily used as an opioid receptor antagonist in the treatment of alcohol and opioid dependence. It works by blocking the euphoric effects of these substances, thereby helping to reduce cravings and prevent relapse.

On the other hand, substances such as LSD, PCP, and psilocybin are all recognized hallucinogens, known for their ability to alter perception, mood, and cognition. These substances can create visual or auditory hallucinations and significantly change a person's sensory experiences. The key distinction lies in their pharmacological effects and the psychological experiences they produce, which are markedly different from those of Vivitrol.