A psychotropic drug or psychotropic compound was a chemical that altered brain function in such a way as to affect mood, consciousness, perception, or behavior. These effects were usually temporary.
On Earth, some Humans used a psychotropic drug known as LSD, which was responsible for impacting the minds of former users negatively later in their life. James T. Kirk, during a time travel mission that visited Earth's past, used this fact in an attempt to explain Spock's unusual comments, behavior, and attire, by stating that he had taken too much "LDS" [sic] during the 1960s while partaking in the Free Speech Movement at Berkeley, California. (Star Trek IV: The Voyage Home)
In 2151 an away team from Enterprise NX-01 fell victim to a naturally occurring psychotropic compound, known as tropolisine on an uninhabited Class M planet later named Archer IV. (ENT: "Strange New World")
Later that year, Captain Jonathan Archer scanned for psychotropic compounds on the rogue planet Dakala, finding none, to convince T'Pol that he was not hallucinating a woman in the jungle. (ENT: "Rogue Planet")
The Ktarian game was stimulating the brain similar to a psychotropic drug. (TNG: "The Game")
An experimental Cardassian psychotropic drug.
Empok Nor had been the site of a failed experiment by the Cardassians to use psychotropic drugs to enhance Cardassian xenophobia, in order to create more effective soldiers. Three members of the Third Battalion, First Order were treated to the drug, and left behind in stasis tubes when the station was abandoned. The two surviving members were awakened in 2373 by a Starfleet salvage team, who killed three members of the team before being killed by Elim Garak, who had also been exposed to the drug. The drug caused Garak to kill a fourth member of the team before he was neutralized by Chief Miles O'Brien. (DS9: "Empok Nor")