The study included data from almost 14,000 Icelandic children born between 1994 and 1996. Data on prescription-medication use were gathered for 2003 through 2008, according to the report.
The researchers identified 1,029 children who were treated with ADHD drugs during the study period. Of those, 96 percent were treated with methylphenidate (brand names Concerta and Ritalin), a stimulant medication.
The academic performance of children without ADHD didn't change much between the fourth and seventh grade tests (given at ages 9 and 12, respectively), the investigators found.
In children who received medication, the researchers noted a decline in academic performance that was concentrated in those who started treatment later. The average decline for those who started treatment later was 9.4 percentage points on the math test, the authors noted. Late treatment was any treatment that began 25 to 36 months after the fourth-grade test.
Overall, those who started treatment late had a 70 percent increased risk of having a decline in math performance between the two tests, and a 10 percent increased risk of having a decline in language arts tests, the investigators found.
Girls who started treatment late had a 2.7 times higher risk of having a decline in their math scores compared to a 40 percent increased risk for boys who started treatment later.
Zoega said the study wasn't designed to look at the reasons behind the decline, but she theorized that the bigger declines in girls' scores with late treatment may be because girls are more likely to have inattention problems, and math skills may be more affected by a lack of focus.
Dr. Andrew Adesman, chief of developmental and behavioral pediatrics at Steven and Alexandra Cohen Children's Medical Center of New York, in New Hyde Park, N.Y., said he wasn't surprised by the st
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