- (figuratively) The one weakness of something or someone that is otherwise invulnerable, an Achilles heel.
1991 October 25, Bryan Miller, “Restaurants: John Clancy's”, in New York Times:
- The terrific seven-grain bread here is the kryptonite of restaurant reviewers: two mini-loaves and you are incapacitated.
2006 March 25, Jonathan Leggett, “Cult musicians”, in The Guardian:
- So while recording or sampling a Charlie Manson track […] makes for shock rock kudos aplenty, a devotion to Hubbard is kryptonite for credibility.