Teenagers who regularly smoke cannabis run the risk of causing lasting damage to their intelligence, memory and attention span. The more people smoke the greater the decline according to a new long-term study of around 1,000 subjects.
The study, carried out in New Zealand by an international team of researchers, is unique because it measured subjects’ IQ at a young age before they used the drug. Some evidence already existed to suggest that marijuana users were less intelligent but it was unclear whether that was the result or the cause of the drug use.
The study followed a group of more than 1,000 people born in 1973 in the southern New Zealand city of Dunedin. Their IQ was measured for the first time at the age of thirteen. They were then tested and interviewed five times in a 20 year period between the ages of 18 and 38. The results of the study, which were published today in the US journal Proceedings of the National Academy of Sciences, show that those who started using cannabis in adolescence and then carried on using it for years showed an average eight-point IQ decline. Stopping or reducing the drug use failed to fully restore the lost IQ.
Those subjects who only began smoking cannabis at a later age did not show a similar reduction in mental ability. Researchers say the results have been corrected to take into account the use of alcohol and other drugs as well as the number of years spent in education.
Reacting to the study,Wim van den Brink,Professor of Psychiatry and Addiction at the University of Amsterdam said it was interesting research but its findings should not be overestimated. Speaking to Dutch daily de Volkskrant he pointed to the results from a sub-group in the study who stopped smoking the week before being tested. The effect on their IQ’s was much less pronounced. “The researchers are right to warn of the consequences of cannabis use at a young age,” he said, “but their results are probably being exaggerated.”