Which substance is classified as a stimulant?

Cocaine is classified as a stimulant because it primarily works by increasing levels of dopamine, a neurotransmitter associated with pleasure and reward, in the brain. This results in heightened energy, increased alertness, and enhanced focus. Stimulants like cocaine act on the central nervous system to boost physical and mental performance, which is in stark contrast to sedatives or depressants that typically slow down the body's systems.

In comparison, heroin is an opioid that produces sedative effects, alcohol is a depressant that slows brain function and impairs motor skills, and marijuana, while it can have stimulant or sedative-like effects depending on the strain and dosage, is not primarily classified as a stimulant. Thus, cocaine stands out as the only true stimulant among the options provided.