A nerve gas is an airborne (gaseous or aerosol) agent of chemical warfare. Usually lethal, these compounds work by inhibiting synaptic activity. Such activity is essential for propagation of of impulses from the central nervous system to other systems of the body; without that activity, the victim becomes paralyzed and dies.
Prior to the 23rd century, the Klingons employed a nerve gas called theragen. Theragan was deadly only in concentrated form. (TOS: "The Tholian Web")
In 2372, when Quark sought to fulfill the terms of a contract with Brunt calling for his death, nerve gas was one option Garak suggested he use to cause it. However, Quark thought the nerve gas smelled bad, and he decided he wanted his death to be sudden. Ultimately, he broke the contract. (DS9: "Body Parts")
In 2373, The Doctor euthanized Captain Kathryn Janeway with nerve gas, in one of the delusional realities induced by a non-corporeal being posing as her father. (VOY: "Coda")