It's no longer shocking to hear of children with attention-deficit hyperactivity disorder—and others simply facing a big test—taking ADHD medicine to boost their performance in school. But new studies point to a problem: There's little evidence that the drugs actually improve academic outcomes.

Stimulants used to treat ADHD like Ritalin and Adderall are sometimes called "cognitive enhancers" because they have been shown in a number of studies to improve attention, concentration and even certain types of memory in...