A nerve gas, nerve agent or neural agent was an airborne (gaseous or aerosol) agent of chemical warfare. Usually lethal, these compounds worked by inhibiting synaptic activity. Such activity was essential for propagation of impulses from the central nervous system to other systems of the body; without that activity, the victim became paralyzed and died.
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 2371, Samantha Wildman and The Doctor found that because Vidiian immune systems had been compromised by the Phage, they could synthesize a neural agent that would incapacitate them without affecting the crew of USS Voyager. When complete, the agent would inhibit the Vidiian's motor functions without causing any permanent damage. When a Vidiian ship later attempted to board Voyager, Captain Kathryn Janeway called down to Sickbay to have them release it on deck 3, to which The Doctor figured 50 milliliters would suffice. (VOY: "Fury")
In 2372, when Quark sought to fulfill the terms of a contract with Brunt calling for his death, nerve gas was one option Elim 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 response to her having had contracted the Vidiian phage, during one of the delusional realities induced by the alien being attempting to lure her into its matrix. (VOY: "Coda")