What's the deal with Kryptonite?

[Orrex]

Does it actually directly harm Superman, or does it just temporarily reduce his invulnerability, or what?

[Spider Jerusalem]

1. Supposedly it's radioactive in a way that specifically affects Kryptonian physiology.

although in current continuity it affects humans over long periods of time, too...Lex Luthor lost a hand to Kryptonite poisoning-induced cancer after wearing a green K ring to keep Superman at bay (from what I remember from all the comics I read back in the late '80's, anyway--I haven't had too much interest in superhero comics for quite a while now).

[Bassic]

2. It harms him. Can even kill him. nt