Neurocine is a chemical that is typically delivered in gaseous form as an anesthetic. High concentrations of neurocine can be fatal (DS9: "Civil Defense"; VOY: "Message in a Bottle")
The Cardassians used lethal concentrations of neurocine as crowd control and as part of a counter-insurgency program aboard Terok Nor. (DS9: "Civil Defense")
Neurocine was also used to incapacitate Romulan hijackers aboard the USS Prometheus in 2374 as well as allow the Cardassian Rebellion to steal a Jem'Hadar attack ship and its Breen weapon in 2375. (VOY: "Message in a Bottle"; DS9: "Tacking Into the Wind")
In 2377, Dr. Alan Murrow assisted the USS Baldwin crew in destroying a Jem'Hadar dreadnought by flooding the ship with neurocine. (USS Baldwin: "Departure")
Neurozine article at Memory Alpha, the canon Star Trek wiki.