It's debatable whether ADHD (No such thing as just "ADD" for the record) actually exists or not. My little brother is classified as "ADHD, predominately hyperactive-impulsive type", but I don't think medicine (which I know not all people with ADHD take) truly helps him.
My opinion of his particular case is that he is just highly (and annoyingly) extraverted and requires more stimulation than sitting in a desk and listening to a teacher has to offer him. Because he can't conform to the school's expectations, they told my mom he can't come to school anymore unless he takes his adderall. Heaven forbid they try and tailor their teaching methods to his learning style!
Sorry, that was my ADHD rant. I feel bad for the guy because his intelligence is never reflected in his grades because of the lack of hands-on learning. Plus he gets in trouble a lot for getting out of his seat.
It's interesting because all the people I know with ADHD tend to be more on the extraverted side, but I assumed that there are introverts diagnosed as well. I wonder if introverts tend to fall more along the lines of "ADHD predominately inattentive type"?