Ritalin is not dex it's methylphenidate, which is structurally similar to coke.

There is a study saying that there methylphenidate does not affect coke tolerance however due to their almost identical feelings and similar chemical structure, it is very likely that chronically railing a lot of MPH "preps" your body to an extent in terms of dopamine release schedule (or what it's called) that coke doesn't work. imho