A psychoactive drug, sometimes called a mind-altering substance or agent, was a mood-altering chemical substance. It primarily affected the central nervous system by altering brain function resulting in changes in behavior, perception, consciousness, and cognition. Psychoactive drugs induced elation and euphoria, hallucinations, or altered sensory experiences.
In 2153, Hoshi Sato asked Doctor Phlox if her feeling of being watched and hearing of whispers possibly stemmed from exposure to a substance which caused hallucinations. Phlox assured her there was no evidence of any mind-altering substances aboard Enterprise NX-01. (ENT: "Exile")
Chakotay revealed to Kathryn Janeway that in the past, Native Americans used psychotropic herbs while embarking on vision quests to induce an altered state of consciousness. With the advent of modern technology, however, they moved away from using drugs to induce hallucinations and instead discovered the akoonah, a device that affected one's mind just like the herbs. (VOY: "The Cloud", "Basics, Part I", "The Fight")