Intelligent children are more likely than their less intelligent peers to use illegal drugs in later life, according to a study which has found a link between high IQ scores and drug misuse.

Children who were in the top third in terms of IQ when aged five and 10 were found to be at significantly increased risk of having taken illegal drugs such as cannabis and cocaine when they became older.

The study was based on interviews with nearly 8,000 people who were part of the 1970 British Cohort Study, which involved measuring IQ scores when each child was five and 10, and asking them about their drug habits when they were 16 and 30.

By the age of 30, around one in three men (35.4 per cent) and one in six women (15.9 per cent) had used cannabis while 8.6 per cent of men and 3.6 per cent of women had used cocaine in the previous 12 months, the study found. A similar pattern of behaviour was found for other drugs, such as barbiturates, LSD and heroin. Boys in the top third in terms of IQ when aged five were about 50 per cent more likely than the bottom third to have used drugs such as amphetamines and ecstasy when aged 30.

The effect was even stronger among women. High-IQ women were more than twice as likely as low-IQ women to have used cannabis and cocaine in the past year, for instance.

James White of Cardiff University, who led the study in the Journal of Epidemiology and Community Health, said that because the effect could be picked up at the age of five, before schooling, it may be independent of education.

"The study just looked at whether you had or had not used drugs in the last year. We don't know the level of usage and we don't know the harm of low-level drug use," Dr White said.