Which substance is known to cause CNS stimulation?

Caffeine is a substance well-known for its ability to stimulate the central nervous system (CNS). It acts primarily as an adenosine receptor antagonist, which means it blocks the action of adenosine, a neurotransmitter that promotes sleep and relaxation. By inhibiting adenosine, caffeine increases neuronal firing and the release of other neurotransmitters such as dopamine and norepinephrine, leading to heightened alertness, improved mood, and increased concentration.

In contrast, alcohol is a depressant that slows down CNS activity, leading to decreased inhibitions and impaired cognitive function. Heroin and fentanyl are opioids, which typically act as CNS depressants, causing sedation and reducing pain perception rather than stimulating the brain. Thus, caffeine’s unique mechanism of stimulating brain activity clearly identifies it as the correct answer among the choices provided.