Which substance is classified as a stimulant?

Cocaine is classified as a stimulant because it primarily works by increasing the levels of dopamine, a neurotransmitter associated with pleasure and reward, in the brain. This action leads to heightened alertness, increased energy, and a sense of euphoria, which are characteristic effects of stimulant substances. Stimulants generally enhance brain activity and can lead to increased heart rate and blood pressure, in addition to stimulating the central nervous system.

In contrast, alcohol is classified as a depressant, as it slows down brain function and can lead to reduced inhibitions and cognitive processing. Heroin, an opioid, is known for its sedative and pain-relieving effects, also falling into the category of depressants. Marijuana has complex effects but is primarily recognized for its relaxing and sometimes hallucinogenic properties, rather than stimulating effects. Therefore, cocaine stands out as the substance classified specifically as a stimulant among the provided options.