A Psychotropic drug is the title of any chemical that alters brain function in such a way as to result in changes to the mood, consciousness, perception, or behaviour of the effected humanoid, these effects are usually temporary.
During the 1960s, Human on Earth willfully used a psychotropic drug known as LSD, which resulted in damaging the minds of those who used it later in their lives. James T. Kirk used this fact in attempt to explain Spocks unusual comments, behavior, and to some degree attire, by stating that he had taken too much of the drug during that era, while practicing the Free Speech Movement at Berkeley, California. (Star Trek IV: The Voyage Home)
In 2151 an away team from the Enterprise (NX-01) fell victim to a naturally occurring psychotropic compound, known as tropolisine on an uninhabited Class M planet. (ENT: "Strange New World")
Empok Nor had been the site of a failed experiment by the Cardassians to use psychotropic drugs to enhance Cardassian xenophobia. Three members of the First Order, Third Battalion, who were left on the station in stasis tubes were altered by the psychotropic drug, two survived. They were stumbled upon in 2373, by a starfleet team looking for salvage. The two surviving Cardassian soldiers were awakened and they killed three members of the starfleet team before they were stopped. Garak was also exposed to the psychotropic drug and killed a fourth member of the starfleet team before he was neutralized by Chief Miles O'Brien. (DS9: "Empok Nor")