Which substance is classified as a narcotic?

Heroin is classified as a narcotic because it is an opioid that acts on the central nervous system to produce pain relief, sedation, and euphoria. It works by binding to specific receptors in the brain that are responsible for controlling pain and feelings of pleasure. As a substance derived from morphine, which is itself an alkaloid found in the opium poppy, heroin is known for its high potential for addiction and abuse. The term "narcotic" is often associated with substances that have soothing effects, but it is also linked to drugs that can dampen the senses and lead to significant alterations in consciousness.

In contrast, other substances listed do not fit the classification of narcotics. Cocaine is a stimulant that increases alertness and energy; PCP is known as a dissociative anesthetic; and caffeine is a mild stimulant commonly found in coffee and tea, which does not provide the pain-relieving properties associated with narcotics. This distinction highlights why heroin is the correct choice in this context.