Originally Posted by dnno1
You have to look at the definition of the word. There are two definitions of the word kryptonite. The first describes pieces of the planet Krypton that have become radio active. The second, which has become a modern day colloquialism is anything that is your weakness. In the film, Supeman's connotative kryptonite was the actual environment of his home world. Once he was immersed in it, his body (as a result of being on Earth for more than three decades) would reject it (as to why he was coughing up blood in one scene and choking in another). It wouldn't kill him, but certainly over time, he would become mortal.
While that is right in regards to MOS, I didn't see it as a "new" kryptonite but more an introduction to the real thing if it's introduced in later films.