WASHINGTON DC, United States?Almost one million children in the United States are potentially misdiagnosed with Attention Deficit-Hyperactivity Disorder (ADHD) because they were the youngest and least mature in their kindergarten classes, a US study released Tuesday found.

The Michigan State University study found that prescriptions for the misdiagnoses could represent spending of $320 million to $500 million a year, with 80 to 90 million of it paid by Medicaid, a public health insurance program for the poor.

The most commonly prescribed drug for ADHD is Ritalin (methylphenidate), a psychostimulant, and its long-term effects are not well known, wrote lead author Todd Elder, of Michigan State University, whose study will appear in the Journal of Health Economics.

Elder studied some 12,000 young children.

He found that "the youngest kindergarteners were 60 percent more likely to be diagnosed with ADHD than the oldest children in the same grade. Similarly, when that group of classmates reached the fifth and eighth grades, the youngest were more than twice as likely to be prescribed stimulants" for ADHD.

Though only doctors diagnose the condition, "many ADHD diagnoses may be driven by teachers' perceptions of poor behavior among the youngest children in a kindergarten classroom," Edler wrote.

"But these 'symptoms' may merely reflect emotional or intellectual immaturity among the youngest students."