By Julie Steenhuysen

CHICAGO, Oct 6 (Reuters) - Girls who take stimulants to treat attention deficit hyperactivity disorder are less likely than others with the condition to start smoking or to abuse alcohol or drugs, U.S. researchers said on Monday.

They said girls with ADHD who took stimulant drugs had half the risk of substance abuse and nearly half the risk of smoking cigarettes as those who were not treated with drugs.

"It shows a greater than 50 percent reduction for developing a substance use," said Dr. Timothy Wilens, director of a pediatric substance abuse program at Massachusetts General Hospital, whose study appears in the Archives of Pediatrics and Adolescent Medicine.

Several studies have found that people with ADHD are far more likely to smoke and abuse drugs, and treatment with stimulants such as Ritalin, known generically as methylphenidate, can lower this risk in boys. But at least one study suggested this benefit did not extend to adolescent girls with ADHD.

Millions of children with ADHD are treated with drugs such as Novartis AG's <NOVN.VX> Ritalin and Shire Plc 's <SHP.L> Adderall. Wilens disclosed financial ties to both of those companies and several others, including Eli Lilly and Co <LLY.N>.

Wilens and colleagues studied 140 girls with ADHD aged 6 to 18 -- 94 percent of whom were taking stimulant medication -- and 120 girls without ADHD. The girls periodically had psychiatric evaluations over five years.

The researchers wanted to see if treating girls with ADHD might increase the risk of smoking and substance abuse. Instead, it lowered the risk.

Because the average age of the girls studied was 16, it is not yet clear whether this decreased risk will continue through young adulthood, when people have a higher risk for smoking and substance abuse.

Wilens said any finding that a medication reduces the odds of smoking is important. "If you smoke and have ADHD, over half of those kids ultimately go on to have a substance abuse problem," he said in a telephone interview.

The research team also included Dr. Joseph Biederman, a Harvard psychiatrist who has been accused by Iowa Republican U.S. Sen. Charles Grassley of not fully disclosing payments from drug companies. Biederman in this study disclosed ties to Novartis, Shire and other companies.

The study was funded by the National Institutes of Health and by the Lilly Foundation.

(Editing by Maggie Fox and Doina Chiacu)

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