Has the effects of Focalin been proven to diminish over time? Can you develop a tolerance to the medication? My 6 1/2 year old son was prescribed Focalin yesterday but needs to see a cardiologist 1st b/c they saw a slight irregularity on his ekg. I have been reading the reviews and am concerned about the long term usage of the medication. Also what is the onset and duration of the medication? Thank you,