Who knows? But there are many things much less severe than drug addiction that can still preclude one from being able to have a successful marriage.
And who's to say that the addiction itself caused the end of the marriage? Generally, those who become addicted want the little vacation that the painkillers provide in order to escape some aspect of their lives. It's just as possible that the marriage was in trouble for some time, and being free of the addiction enabled Rush to see a little more clearly what wasn't working in his life.
Without question. These sort of failures are often the product of many core weaknesses. Seldom is one party or the other completely to blame. I'd say physical abuse is one such caveat to that, but those are relatively rare in todays very abuse-conscious America.
We'll never know what truly transpired between Marta and Rush. I'd prefer not to judge either, but proclaim them human and incompatible, forgive them, and move on.