CHICAGO (Reuters) - A common stimulant drug used to treat attention deficit hyperactivity disorder can cause brain changes in mice similar to those seen in cocaine addiction, U.S. researchers said on Monday.

They said healthy mice exposed to daily injections of the Novartis drug Ritalin, or methylphenidate, developed changes in the reward centers of their brains, and some of these changes resembled those in mice given cocaine.

“Methylphenidate, which is thought to be a fairly innocuous compound, can have structural and biochemical effects in some regions of the brain that can be even greater than those of cocaine,” Dr. Yong Kim of Rockefeller University in New York, whose study was published in the Proceedings of the National Academy of Sciences, said in a statement.

The study, funded by the National Institute on Drug Abuse, follows a number of studies in humans that have found the drugs to be safe when used to treat ADHD.

It was prompted by reports suggesting that more than 7 million people in the United States have abused methylphenidate, using it to get high or to improve academic performance.

Dr. Nora Volkow, director of the National Institute on Drug Abuse, said in a telephone interview that very little is known about the effects of these drugs when used to improve concentration in healthy people.

“What this study shows was in these animals exposure of two weeks to methylphenidate actually produced changes that are comparable to what is seen with chronic exposure to cocaine,” Volkow said.

Millions of children take stimulants such as Ritalin and Shire Plc’s Adderall to treat ADHD, a condition marked by restlessness, impulsiveness, inattention and distractibility that can interfere with a child’s ability to pay attention in school and maintain social relationships.

Volkow stressed that studies in adolescents show methylphenidate does not increase the risk for later addiction, and several have found adolescents with ADHD are far more likely to smoke or abuse drugs, and treatment with stimulants such as Ritalin can lower this risk.

“If you don’t treat them as adolescents, actually you may be seeing more abuse of substances,” Volkow said.

But she said nonmedical use of methylphenidate and other stimulants may lead to addiction.

Part of the reason there has been an increase in the abuse of stimulant medications has to do with the notion that they are less dangerous than illicit substances, she said.

“This is wrong. They can be as dangerous as illicit substances when used inappropriately.”