Up to a fifth of adults, including college students and shift workers, may be using cognitive enhancers, a poll of 1,400 by Nature journal suggests.

Neuropsychologist Professor Barbara Sahakian of Cambridge University said safety evidence is urgently needed.

Experts gather to debate this topic at a meeting in London on Monday evening.

Professor Sahakian's own work shows 17% of students in some US universities admit to using the stimulant Ritalin (methylphenidate) - a drug designed to treat hyperactive children - to maximise their learning power.

One in five of the 1,400 people who responded to the Nature survey said they had taken Ritalin, Provigil (modafinil) or beta-blockers for non-medical reasons. They used them to stimulate focus, concentration or memory.