Maybe Kryptonite is like alcohol -- he develops a tolerance for it. First exposure -- highly intoxicating (or toxic, in this case). After that, there's less reaction, unless there's been a long time between exposures, at which point the body loses some of its tolerance.
"Oh, you can’t help that," said the Cat: "we’re all mad here. I’m mad. You’re mad."
"How do you know I’m mad?" said Alice.
"You must be," said the Cat, "or you wouldn’t have come here.”
- Lewis Carroll, Alice in Wonderland
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