Ask my doctor, he showed me some literature on the topic prior to prescribing. I was, naturally, very leery of taking any medication at the time.
Normal people taking ritalin had disruptions, but ADD people taking ritalin had modifications that got their 'brainprints' looking quite similar to normal people's.
Ritalin stimulates one particular area in the brain that is underutilized in ADD people. If there is no need for the ritalin, it does not concentrate there but instead causes a general stimulant effect to the entire brain. This is why normal people get an amphetamine result from taking ritalin, and ADD people do not.
Look here for a few papers on dopamine uptake and ADHD.