In 2007 there were four million children diagnosed with ADHD in the U.S.
Two out of three were taking medication to treat the condition.
However, medication rates varied significantly between states.
In Nevada, a third of kids diagnosed with ADHD were treated with medication, compared to eighty percent of those in Mississippi.
Researchers also found boys were medicated more often than girls.
Experts say the uneven distribution of ADHD meds throughout the U.S. may be contributing to shortages in some areas.