Does Cannabis Diminish Intelligence?
Adolescents who smoke cannabis may have impaired intellectual capacity, memory and concentration.
This is the result of a study carried out in New Zealand. An international group of experts monitored the IQ development of 1,000 people. The research conducted on these people is unique because the study started before the young people started smoking cannabis, marijuana or hashish. Previously it had been suspected that cannabis smokers were less intelligent, but it was not known whether this was the cause of their use of the soft drug or the result of their use.
The 1,000 people studied were born in the New Zealand city of Dunedin in 1973 and the first intelligence test was given to a 13-year-old boy before he started smoking the drug. They were subsequently tested and interviewed five times over a period of 20 years, between the ages of 18 and 38. The study, published on Monday in the US journal Proceedings of the National Academy of Sciences, found that prolonged cannabis use during adolescence leads to an eight-point reduction in IQ. Permanently stopping or reducing consumption does not result in the full recovery of lost intelligence.
People who started smoking hashish or marijuana at an older age do not show a similar reduction in intellectual capacity. The researchers claim that the results have taken into account whether the individual studied also consumed alcohol or other drugs. When testing intelligence, the user's school performance was also taken into account.
However, Wim van der Bink, professor of psychiatry and addiction at the University of Amsterdam, the Netherlands, said the study is interesting but its results should not be overestimated.
In a statement to Dutch newspaper De Volkskrant, van der Brink added that a subgroup of the people studied stopped smoking a week before the last interview and that their IQ scores were considerably lower. Van der Brink said researchers are right to warn against early cannabis use, but added that the results have probably been exaggerated.