Which of the following is classified as a narcotic?

Heroin is classified as a narcotic because it is an opioid that acts directly on the brain's receptors to produce pain relief, relaxation, and euphoria. Narcotics are typically substances that can induce sleep and reduce pain, and heroin fits this definition due to its ability to mimic endorphins in the body, leading to significant effects on the central nervous system.

In contrast, cocaine is a stimulant that increases energy and alertness rather than having sedative qualities typical of narcotics. Alcohol, while it can produce sedative effects, is classified as a depressant and is not a narcotic. Marijuana, despite having psychoactive effects, is primarily classified as a cannabinoid rather than a narcotic. Understanding these classifications helps clarify why heroin is categorized as a narcotic while the others are not.