CHICAGO (Reuters) - Parents of kids on prescription drugs for attention deficit hyperactivity disorder are 10 times more likely to be on such a medication themselves, according to an analysis of U.S. prescription use released on Thursday.

Research by the prescription drug manager Medco Health Solutions Inc., which runs employee health plans for companies, examined prescription claims of 107,000 children and their parents during 2005.

In about 60 percent of the cases where child and parent were on ADHD medication, the child went on the prescription first, the study found.

Adult ADHD usually excludes the "H" -- hyperactivity -- according to doctors. That may be why it is less diagnosed in adults. It is also significantly less diagnosed in young girls, who tend not to be as hyperactive than boys.

"Moms are catching up in adulthood," said Robert Epstein, chief medical officer at Medco, which buys prescription drugs in bulk for about 55 million people.

Although the condition is more common in children, heredity does play a role and ADHD can linger into adulthood, according to experts.

"In adults, the hyperactivity part is gone, but the inattentive part doesn't go away," said Adelaide Robb, a child psychiatrist at Childrens' National Medical Center in Washington, D.C., which is not affiliated with the study.

Franklin Lakes, New Jersey-based Medco, which was spun off of drug maker Merck & Co. several years ago, says its analysis is the first to examine treatment trends, and that its findings give further backing to the role of genetics.

About 8 percent of children are estimated to have ADHD, according to the U.S. Centers for Disease Control and Prevention. About 25 percent of close relatives of kids with ADHD also have the disorder, which is marked by impulsiveness, hyperactivity and inattention.

About 2.5 million children have taken ADHD prescription drugs, the most widely known being Novartis AG's Ritalin, according to the CDC.

The study also looked at about 17,500 identical and nonidentical twins, and found that when one twin is on an ADHD medication, the other twin is much more likely to be on a prescription, compared to non-twin siblings.

The U.S. Food and Drug Administration last month asked makers of all stimulants used to treat ADHD drugs to include warnings of possible risk of sudden death and serious heart problems.

Other ADHD stimulants include GlaxoSmithKline Plc's Dexedrine, Shire Plc's Adderall, Novartis AG's Ritalin and Johnson & Johnson's Concerta.

Cheaper generic versions are available but most have to be taken several times a day, leading parents to prefer the brand-name drugs, Epstein said.

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