Which of these substances is commonly used as a CNS stimulant?

Caffeine is a commonly recognized central nervous system (CNS) stimulant that works primarily by blocking adenosine receptors in the brain. This action increases the release of neurotransmitters such as dopamine and norepinephrine, which enhances alertness, improves mood, and boosts energy levels. Caffeine's stimulating effects contribute to its widespread use in various forms, including coffee, tea, energy drinks, and supplements.

In contrast, the other substances listed do not function as stimulants. Alcohol is classified as a depressant, which means it generally slows down the central nervous system and can lead to relaxation and sedation. Barbiturates are another category of depressants that can have sedative effects, typically used in medical settings for anxiety or seizure management. Opioids, like morphine and codeine, are known for their pain-relieving properties and are also categorized as depressants, affecting the CNS by slowing down its activity.

Understanding these classifications helps clarify why caffeine is identified as a CNS stimulant, while the other substances serve different purposes in the context of CNS effects.