(HealthDay News) – The youngest children in a classroom are significantly more likely to be diagnosed with attention-deficit/hyperactivity disorder (ADHD) and prescribed medication than their older peers in the same grade, according to a study published online March 5 in CMAJ, the journal of the Canadian Medical Association.

Richard L. Morrow, of the University of British Columbia in Vancouver, Canada, and colleagues used data from 937,943 children in British Columbia who were between 6 and 12 years of age between Dec. 1, 1997, and Nov. 30, 2008, to investigate the influence of relative age on the diagnosis and treatment of ADHD in children. The risk of receiving a diagnosis and prescription for medication (including methylphenidate, dextroamphetamine, mixed amphetamine salts, or atomoxetine) was compared for children born in December with those born in January.

The researchers found that, compared with boys and girls born in January, those who were born in December were 30% and 70% more likely to receive a diagnosis of ADHD. Compared with those born in January, boys were 41% more likely and girls were 77% more likely to be given a prescription for a medication to treat ADHD if they were born in December.

“The results of our analyses show a relative-age effect in the diagnosis and treatment of ADHD in children aged 6–12 years in British Columbia,” the authors write. “These findings raise concerns about the potential harms of overdiagnosis and overprescribing.”

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