Narcotics are naturally occurring or synthetic drugs with psychoactive and highly intoxicating properties. Narcotics are often highly addictive and taking them results in changes in perception, mood, consciousness, cognition, and behavior.
During the mid-21st century various Earth militaries controlled their soldiers through the use of drugs which produced a narcotic-like stupor. (TNG: "Encounter at Farpoint")
Trellium-D has some characteristics of narcotics on Vulcans, causing them to lose emotional control, eventually leading to brain damage and death, although it is an unpleasant experience, at least one Vulcan- T'Pol became addicted to the effect.
In 2254 Vina described the Talosian's addiction to experiencing events in the thought record by comparing it to a narcotic. (TOS: "The Menagerie, Part I", "The Menagerie, Part II")
The inhabitants of the failed Federation colony Turkana IV often turned to narcotics as an escape from the great poverty and violence that existed there. Natasha Yar described her knowledge of the effects of narcotic drugs, from experiences she witnessed on her homeworld, to her fellow crewmates, Data and Wesley Crusher in 2364; they did not understand why anyone "would voluntarily become dependent upon a chemical." (TNG: "Symbiosis")
The Ornarans of planet Ornara suffered from an addiction to the narcotic created by the Brekkians known as felicium circa 2364. (TNG: "Symbiosis")
During the Occupation, Bajoran slave laborers on Terok Nor often partook of narcotic drugs, reffered to as "chemicals", it was apparently common for users to be passed out on the Promenade, and for life-threatening disputes over them to arise. Users were often viewed with disgust by other Bajorans, believing that their undignified behavior justified the stereotypes Cardassians had of Bajorans, and the Resistance refused to smuggle them off the station. (DS9: "Things Past")
Cadet-Captain Tim Watters also took narcotics over the course of his self-imposed mission during the Dominion War, in order to deal with the pressure and stress of his duties. As such, many of his actions and command decisions were quite irrational. Also, under the influence of the drugs, appeared to develop the egomanical impression that both he and his crew were "special" and fated to a great destiny. Watters also appeared to develop faith in a higher power guiding his destiny, even broaching the opinion that Jake Sisko was guided to him by it to write his story. The extent to which Watter's actions and beliefs were due to the drugs, stress or his own free choice to avoid returning to the Federation and remain in command was never made clear. (DS9: "Valiant")
Ketracel-white, used by the Dominion to control Jem'Hadar soldiers in the 24th century, shared many of the same pharmacological traits as a narcotic. (DS9: "The Abandoned", et al.; Star Trek: Insurrection)