Which of the following is classified as a stimulant?

Caffeine is classified as a stimulant because it acts on the central nervous system to increase alertness, boost energy, and improve concentration. It accomplishes this by blocking the action of adenosine, a neurotransmitter that promotes sleep and relaxation, thereby promoting a state of wakefulness.

In contrast, alcohol is a depressant, meaning it slows down brain activity and has a calming effect on the body. Barbiturates are also classified as depressants, as they can enhance the effects of neurotransmitters that inhibit brain activity, leading to sedation. Heroin, an opioid, primarily acts as a depressant as well, producing pain relief and a sense of euphoria, but it can also slow breathing and heart rate.

Each of these substances has different effects on the central nervous system, making caffeine the only stimulant among the options listed.