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The maximum therapeutic levels are going to be easy to find for all the different forms of (dex)methylphenidate (D/R/LisDex)Amphetamine and so on.
Regarding speed of different kinds, the body can tolerate very large doses compared to say, Opiates which have a comparatively narrow window between being high and being dead. That's not to say that there is no damage done to the body over time. Bear in mind that methylphenidate and its analogs cause more tachychardia that amphetamines in most cases.