More than one in 20 college students misuses drugs intended to treat attention-deficit/hyperactivity disorder (ADHD), but those who do so typically use the drugs as a study aid, not to get high, researchers found.

A study undertaken by researchers from the University of Michigan, Duke University, and University of North Carolina at Greensboro found that about 5 percent of 3,407 students surveyed said they had used an ADHD medication like Ritalin, Adderall and Concerta without a prescription within the past six-months. Nine percent reporting using ADHD drugs since starting college.

Ninety percent of those who used the medications said the reasons was to help them study, and of those, 90 percent said it was effective.

"The nonmedical use of prescription stimulants among U.S. college students is now at its highest level in 15 years," said Sean McCabe of the University of Michigan, a co-author of the study.

Previous research showed that more than 90 percent of college students who use ADHD medications obtain the drugs from friends.

Students who reported using alcohol and other substances frequently during the past six months were more likely to use ADHD medication without a prescription. Sleep difficulties, irritability and headaches were among the adverse effects reported by students who used the medications.

"Learning about the benefits that students perceive from nonmedical ADHD medication use may inform efforts to prevent this behavior," said David Rabiner, lead study author and senior research scientist at Duke.

The study appears online in the Journal of Attention Disorders.