SLAYER KRYPTONITE. Buffyverse Term/Concept/Idea/Intratext.

In the third season episode "Helpless," Willow, Xander and Oz try to help Buffy understand why her Slayer powers have disappeared. Later they learn that Giles has been injecting Buffy with "an organic compound of muscle relaxants and adrenal suppressers" as part of a test ordered in secret by The Watcher's Council. However, before that, Oz & Xander speculate that what they were looking for was a substance akin to the fictional Superman's reaction to a radioactive element referred to in those stories as Kryptonite.

In the Superman mythos (created by Jerry Siegel and Joe Shuster and owned by DC Comics Inc.) kryptonite is the debris from Superman's doomed planet Krypton. Some of the meteorite fragments followed Superman to Earth and when found can be used as a weapon against Superman, because the radiation causes him to rapidly lose all his powers and saps him of his life force.

Oz & Xander speculate that the same thing is happening to Buffy--that there is some kind of radioactive object in the vicinity which is causing Buffy to lose her Slayer abilities. However, this then deteriorates into an argument between them as to which color of Kryptonite actually saps Superman of his powers and which just causes strange mutations, which leads Willow to demand the boys return to reality.

There is no known Slayer kryptonite--no known object or element that causes Slayers to weaken in its presence and get all melodramatic death sceney.

--Zach Garland