Which substance is commonly classified as a narcotic?

Heroin is classified as a narcotic because it is an opioid derived from morphine, which is synthesized from the opium poppy. Narcotics are substances that have sedative effects and can produce a state of euphoria, along with pain relief. They are often prescribed for pain management but come with a high potential for abuse and addiction due to their effects on the central nervous system. Heroin specifically acts on the brain's opioid receptors, leading to a significant alteration in pain perception and mood, which aligns with the definition of narcotic substances.

In contrast, caffeine, nicotine, and MDMA do not fall under the narcotic classification. Caffeine is a stimulant that increases alertness and energy. Nicotine is a stimulant found in tobacco that can also have calming effects but is not classified as a narcotic. MDMA, known as ecstasy, is primarily recognized for its stimulant and empathogenic effects rather than the sedative properties typical of narcotics. These distinctions highlight why heroin is the correct answer as a commonly classified narcotic.