Neurocine gas was a toxic gaseous compound used by the Cardassians. It was lethal to both Bajorans and Humans.
During the Cardassian Occupation of Bajor, this gas was used as a method of riot control against the Bajoran workers on Terok Nor.
In 2371, when a counter-insurgency program was accidentally activated by the Starfleet crew of the space station Deep Space 9, the ore processing center was flooded with neurocine gas in an attempt to control a worker riot. Later on, the program threatened to flood the entire habitat ring with the gas, potentially killing hundreds of innocent civilians. (DS9: "Civil Defense")
The public explanation for Section 31's recruitment of William Boimler into the organization was that he had died in a neurocine gas leak aboard the USS Titan. (LD: "Crisis Point 2: Paradoxus")