Marijuana is the most commonly used illicit drug in the United States. Its main psychoactive chemical is delta-9-tetrahydro-cannabinol, or THC. Due to the variety of effects it can produce, marijuana can be categorized as a stimulant, depressant, or hallucinogen.ts main psychoactive chemical is delta-9-tetrahydro-cannabinol, or THC. Due to the variety of effects it can produce, marijuana can be categorized as a stimulant, depressant, or hallucinogen.