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 …