Which of the following substances is considered a central nervous system depressant?

Barbiturates are classified as central nervous system depressants. These substances work by enhancing the effect of the neurotransmitter gamma-aminobutyric acid (GABA), which leads to sedation, relaxation, and, in higher doses, can result in a significant decrease in neurological function. They are commonly used in medicine for anxiety relief and as sleep aids, but they can also lead to dependency and overdose, especially when misused.

In contrast, caffeine is a stimulant that increases alertness and energy, amphetamines also stimulate the central nervous system and heighten energy and focus, while cocaine acts as a powerful stimulant that can elevate mood and energy levels. None of these substances have the depressant characteristics that are inherent to barbiturates.