I do not believe it is ever really specified. I know at the end of season 7(I believe that is the season) where she is cured from Braniac's sickness/disease. She leaves a tape for Clark telling him that the world needed him more, and not to come and find her.
Now, I know that in one of the comic book series that I read sometime ago. That her and Clark find that they are better friends then lovers, and she pops in here and there.
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Lana Lang
Lana Lang is a character in the Superman comics and on the show Smallville. She is a cute girl and the object of Clark's affection.
Lana Lang becomes herself however according to some variations of the Superman mythos, she marries Pete Ross.
Joely Fisher provided the voice of Lana Lang in episodes "My Girl" and "The Late Mr. Kent" .
Annette O'Toole