In the March 2006 issue of Pediatrics, a study led by Dr. Tim Wilens found that people with ADHD were more likely to abuse their stimulant medication than people without ADHD. (One could argue that it could be due to the fact that the medication is much more easily accessed by people with ADHD.)
Eleven percent admitted selling their stimulant meds. All of the people who sold their meds also had substance abuse disorder or conduct disorder. Eighty-three percent of those who reported misusing their drugs had another disorder along with ADHD.