Which of the following is NOT considered a CNS stimulant?

Acamprosate is not considered a central nervous system (CNS) stimulant. It is primarily used in the treatment of alcohol use disorder and works by modulating neurotransmission in the brain, particularly glutamate and GABA (gamma-aminobutyric acid) systems. Instead of stimulating the CNS, acamprosate helps to stabilize brain activity and reduces cravings for alcohol. This is fundamentally different from the other substances listed, which act to increase CNS activity and arousal. Caffeine, nicotine, and MDMA all have stimulant properties, as they enhance alertness, energy, and mood, acting on various neurotransmitter systems to produce these effects. Thus, acamprosate is rightly identified as the substance that does not belong in the category of CNS stimulants.