Ritalin, a drug commonly used to treat children with attention-deficit hyperactivity disorder (ADHD), caused changes in the brain cells of mice similar to those seen with cocaine, a new study shows.

The researchers, from The Rockefeller University in New York City, said the findings suggest that chronic exposure to Ritalin in high doses could prove addictive, and highlight the need for more research into its long-term effects.

However, one ADHD expert said he doubted the findings were applicable to children with the condition because the doses used in the study were so high.

Indeed, in experiments with mice, the researchers found that Ritalin in doses higher than those prescribed to treat ADHD caused changes in the reward region of the brain in a way comparable to cocaine. Ritalin and cocaine are both psycho-stimulants, the researchers noted.

"Methylphenidate [Ritalin] and cocaine have similar chemical structures and their pharmacological effects appear to be similar," said study author Yong Kim, a senior research associate at The Rockefeller University.

The findings were published in this week's online issue of the Proceedings of the National Academy of Sciences.

In their experiments, Kim and his colleagues compared the effects of chronic exposure to Ritalin and cocaine in specific reward-related brain regions of mice.

Over two weeks, mice were given daily injections of Ritalin or cocaine. The researchers looked for changes in dendritic spine formation, which is related to the formation of synapses and communication between nerve cells. They also looked for levels of a protein, delta Fos B, which is associated with long-term actions of addictive drugs.

"The results indicate that chronic exposure of methylphenidate, like cocaine, changes neuronal morphology and brain chemistry, but the precise pattern observed is distinct from that of cocaine," Kim said.

The changes in brain chemistry are known to be associated with the process of drug addiction, Kim added.

"Our results show addictive properties of methylphenidate, and imply that chronic exposure of methylphenidate in humans may lead to addiction," he said.

Kim noted that the researchers used higher doses of Ritalin than those prescribed to people. Also, taking the drug orally may not lead to addiction. But, if it's taken in high doses or injected, Ritalin could become addictive, he said.

"Indeed, methylphenidate is widely abused for improving concentration and enhancing performance, or for recreational purposes," Kim said.

But Dr. Jon A. Shaw, director of child and adolescent psychiatry at the University of Miami School of Medicine, thinks there's a greater risk of later substance abuse among children with ADHD who aren't treated with Ritalin.

"There is a lot of evidence that the use of Ritalin decreases the risk of substance abuse and cocaine use in adult life," Shaw said. "Untreated ADHD subjects are at higher risk for substance abuse and misuse of drugs in adulthood than treated ADHD [subjects]," he noted.

Shaw also noted that the mice in the study were given high doses of Ritalin. "Generalizing these findings to adults makes no sense," he said.

Shaw said he has seen only two cases of addiction to Ritalin. In both, the patients were taking higher-than-normal doses and taking them more often than prescribed.

A study published in March 2008 in the American Journal of Psychiatry found that children who are prescribed psycho-stimulants for ADHD are no more likely than their peers to abuse drugs and alcohol as young adults.