Students at Oxford University are believed to be taking drugs designed to tackle hyperactivity in children in a bid to boost exam performance.

Ritalin, a prescription-only drug which is given to youngsters with attention deficit hyperactivity disorder (ADHD), is thought to be used by students to improve concentration during study.

Anecdotal evidence suggests that as many as one in every 19 students at the university has used it during the exam period, but this is a figure the students' union says it cannot be certain is accurate.

Louise Randall, vice president of welfare at the students' union, said: "It is understood that some students may have been using Ritalin to aid study performance.

"Statistics in a recent article claimed that one in 19 may have used Ritalin, but we cannot be sure to the extent of its accuracy.

"Neither my colleagues nor myself have ever dealt with students coming to us for advice on Ritalin. Certainly some students are using drugs but what or why is almost impossible to ascertain because of the furtive nature of the subject."

She added: "Even if a few students are taking unprescribed Ritalin, this is of great concern to us."

An Oxford University final year student, who wished to remain anonymous, said: "While I have never taken Ritalin to help me through exams, I have heard of a few people doing so."

Ritalin is given to hyperactive children to improve their concentration.