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.
Even though trellium-D, a deadly neurotoxin to Vulcans, is not a narcotic, it shares some similarities with narcotics. While gradually degrading the neural pathways of Vulcans, its immediate effects are in removing emotional inhibitions. Over time, use of trellium-D induces insanity and violent rages and the neurological damage becomes irreversible, eventually leading to death. (ENT: "Impulse") T'Pol suffered severe exposure to trellium-D while in an asteroid field and even though Doctor Phlox was able to reverse the neurological damage, T'Pol continued to hunger for the release from the constraints of logic the trellium had allowed. She developed a method of injecting it into her body and became addicted until, with Phlox's help, she successfully withdrew from the substance. However, her ability to control her emotions to a normal Vulcan level was permanently damaged. (ENT: "Damage")
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")
In 2373, when the crew of the USS Voyager docked at Bahrat's space station shortly before entering the Nekrit Expanse in the Delta Quadrant, Chakotay and Tom Paris were offered Rhuludian crystals for sale by a trader named Sutok. He explained that ingesting just one of the crystals would make days of tedious travel seem like moments of exquisite rapture. Chakotay and Paris immediately turned down his offer. (VOY: "Fair Trade")
During the Occupation, Bajoran slave laborers on Terok Nor often partook of narcotic drugs, referred 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 ego-maniacal 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)