Anesthizine was a fast-acting anesthetic, which could be administered through environmental systems as a gas, or through a hypospray as a liquid. It was considered among "the best," by the EMH Mark II along with axonol and neurozine, during the late 24th century. (VOY: "Message in a Bottle") A concentration of seventy parts per million of the gas was typically enough to put a humanoid to sleep. (TNG: "The Hunted")
While attempting to subdue Roga Danar in 2366, Jean-Luc Picard ordered that all the cargo bays on Deck 38 be flooded with anesthizine. After flooding the bays with the gas, it was thought that Roga had escaped the effects of the gas by exiting the bay via use of a pressure suit. In reality, he merely hid in the bay and attacked Worf after the gas was removed. (TNG: "The Hunted")
While attempting to incapacitate the three alien-possessed USS Enterprise-D crew that was holding Ten Forward hostage in 2368, Riker suggested flooding the air vents with anesthizine gas. La Forge, however, turned the idea down, noting that the gas wouldn't affect Data. (TNG: "Power Play")
In 2369, when the Ferengi commandeered the USS Enterprise-D, Ro Laren suggested that they could flood the ship with anesthizine gas, as an option to retake the ship. Jean-Luc Picard turned the option down, however, noting that access to the particular system needed to administer the gas was locked out along with the ships other command functions. (TNG: "Rascals")
The besieging General Krim, of the Bajoran Militia, attempted to flood the conduits of Deep Space 9 with anesthizine gas, starting with level five, in attempt to flush out the insurgents, led by Benjamin Sisko, who were stowed away on the station following its evacuation in 2370. (DS9: "The Siege")
When Keiko O'Brien was possessed by a Pah-wraith in 2373, Miles O'Brien asked the computer how long it would take anesthizine gas to render Keiko unconscious. The computer responded that it would take 1.4 seconds, which was far too long to keep the Pah-wraith from killing her. (DS9: "The Assignment")
When the USS Prometheus was commandeered by the Romulans in 2374, The Doctor attempted to devise a means that they could incapacitate their captors by the use of anesthetics. Offered the options of axonol, neurozine, and anesthizine by the EMH Mark II, The Doctor ultimately chose neurozine as the best choice. (VOY: "Message in a Bottle")
Later that year, Sisko flooded all compartments of the USS Defiant, except the engine room, with anesthizine gas after the crew managed to commandeer engineering from Jem'Hadar control. (DS9: "One Little Ship")
This term has many spelling variation in the entire Star Trek series, as at least four different spellings have been used by the writers in describing this gas in various scripts. In "The Hunted", it was spelled "anestazine"; in "Power Play", "Rascals", and "The Siege", it was spelled "anesthizine"; in "One Little Ship", "Message in a Bottle", and "Critical Care", it was spelled "anesthezine"; and in "The Assignment", it was spelled "anesthetine."
"Anesthizine" is the spelling choice used here because it is the spelling of choice used by StarTrek.com. The Star Trek Encyclopedia uses the spelling anesthezine.