A fast-acting anaesthetic gas that Starfleet crews release through a ship's ventilation system to subdue intruders or violent individuals in a sealed compartment without resorting to weapons fire.
Anesthezine is a sedative gas carried aboard Federation starships and stations. Because it can be pumped into a single room or deck through the environmental systems, it is treated less as a medicine than as a security measure: a way of knocking out everyone in a compartment without firing a phaser.
In The Next Generation it is one of the tools the Enterprise-D crew reach for in The Hunted, when the Angosian super-soldier Roga Danar proves almost impossible to hold, and it is discussed again in hostage situations such as Power Play, where the risk to the captives has to be weighed against the speed of the gas.
The substance is mentioned in several later series as well, usually with the same caveat: it works on most humanoids, but not on every species, and not on holograms or androids.