Which of the following is a common example of a stimulant?

Cocaine is a well-known example of a stimulant due to its effects on the central nervous system. Stimulants are substances that increase activity in the brain, leading to elevated mood, increased energy, and heightened alertness. Cocaine specifically works by blocking the reuptake of certain neurotransmitters, particularly dopamine, which results in an increase in their levels in the brain. This mechanism is responsible for the euphoric effects commonly associated with cocaine use.

In contrast, Xanax and Valium are both classified as benzodiazepines, which are sedatives used primarily to treat anxiety and insomnia. They have a calming effect on the central nervous system rather than a stimulating one. Heroin, on the other hand, is an opioid that depresses the central nervous system, leading to a reduction in pain and often a sense of euphoria, but does not increase alertness or energy, characteristics associated with stimulants. Therefore, cocaine stands out as the correct choice in this context.