KRYPTONITE. BtVS Intertext/Allusion.
Kryptonite is a fictional element from the Supermanverse created by Jerry Siegal and Joe Shuster. Remnants of Superman’s home planet Krypton formed rock, metal, or crystalline fragments after it was destroyed by an explosion, creating distinct types of kryptonite. Each emits radiation that has varying effects on the surviving Kryptonians.
In “Helpless," Willow, Buffy, Xander, and Oz speculate on the cause of Buffy’s sudden loss of Slayer powers, but Xander and Oz become distracted as they argue about the effects of various types of kryptonite.
Xander: You know, maybe we’re on the wrong track with the whole spell, curse and whammy thing. Maybe what we should be looking for is something like, um, Slayer kryptonite.
Oz: Faulty metaphor. Kryptonite kills.
Xander: You’re assuming I meant the green kryptonite. I was referring of course, to the red kryptonite, which drains Superman of his powers.
Oz: Wrong. The gold kryptonite’s the power sucker. The red kryptonite mutates Superman into some sort of weird . . .
According to the Encyclopaedia of Superman, green kryptonite, the most common type, produces symptoms of extreme fatigue in Superman and is potentially fatal. Red kryptonite produces unpredictable, temporary effects, including duplication and transformation into other creatures, while gold kryptonite causes Superman to permanently lose his powers.
In “Superstar," kryptonite is again evoked as an analogy by Xander to explain why Jonathan loses his “super powers” when faced by a demon.
Jonathan: I do have a history with the creature. The monster. The problem is, every time I face it my mind becomes sort of confused. There’s some kind of power it possesses.
Xander: Oh!! Oh!! He’s like your kryptonite.
The effects of kryptonite are also referred to in “First Date” as Andrew tries to trick the First-as- Jonathan into revealing any vulnerabilities.
Andrew: Okay. Say – um, so you have any weaknesses I should know about if I’m gonna work for you like, uh, kryptonite or allergies?