Take Ritalin for fun and you run the risk of addiction. That's if the drug causes the same chemical and structural changes in human brains as it does in mice.

Ritalin is prescribed to children with hyperactivity disorders, but many American teenagers also take it without a prescription to boost academic performance, or for pleasure.

When Yonk Kim and his colleagues at the Rockefeller University in New York gave mice the drug for a fortnight, a greater number of spiny neurons formed in the nucleus accumbens, a brain region stimulated by all addictive drugs (Proceedings of the National Academy of Sciences, DOI: link). "These changes in neuronal structure and brain chemistry are known to be associated with the process of drug addiction," warns Kim.

The finding is backed up by previous studies that found signs of addiction in recreational users. In contrast, hyperactive children prescribed the drug don't usually show signs of addiction.