Is heroin a type of hallucinogen?

Yes, heroin is a psychoactive drug. The DEA classifies it as a "Schedule-1" drug which also includes hallucinogenics as LSD, PCP, etc. Pharmacologically, heroin and methadone are both "pro-drugs" which metabolize to opiate-like molecules like morphine in the body. Note that the term "psychoactive" does not medically indicate a drug of an illegal or recreational nature. Many Schedule 2,3,4 drugs are also psychoactive in nature, and include morphine, Demerol, Xanax, codeine, barbiturates like Seconal, Talwin, etc. This is simply a term that describes these agents as having a pronounced stimulant or inhibitory effect on the CNS (central nervous system) in the body.