Which of the following is NOT one of the four major classes of psychoactive substances?

The classification of psychoactive substances typically includes four major categories: stimulants, narcotics, hallucinogens, and depressants. Stimulants, such as caffeine and amphetamines, enhance alertness and energy. Narcotics, often referred to as opioids, are primarily used for pain relief and include substances like morphine and heroin. Hallucinogens, including substances like LSD and psilocybin, alter perception and can lead to visual and auditory hallucinations.

Antidepressants, while they are psychoactive and affect mood and emotional state, are not considered a major class of substances in the context of psychoactive substances. They are a classification of medication often prescribed for depression and anxiety disorders, falling under the broader category of pharmaceutical substances but not forming a distinct class when grouping psychoactive substances.

This distinction is important for understanding the landscape of psychoactive substances and recognizing that while antidepressants influence mental states, they don't fit within the primary categories used for classifying other recreational or therapeutic drugs.