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Sunday 21st July 2019

High-school cannabis use linked to lower IQ

28th August 2012

Smoking cannabis under the age of 18 may affect brain development, as the substance has been associated with a significant and irreversible drop in IQ levels among smokers who start using the drug in secondary school.


Researchers followed around 1,000 people in New Zealand from childhood onwards over the course of two-and-a-half decades, testing their IQ before any of them had ever tried cannabis.

They found that young people who used the drug before their brains had fully matured at the age of 18 lost several IQ points compared with their pre-cannabis scores.

Anecdotal evidence has suggested that people who use cannabis regularly do not always attain their full potential.

Now, the study carried out by an international team of researchers in the New Zealand town of Dunedin may explain why.

The study participants were assessed as children before any of them had used cannabis, and revisited for interviews a number of times until the age of 38.

Researchers found that those who used cannabis regularly and persistently at any age showed a drop in their IQ levels, even when other factors like the use of other drugs, including dependency on alcohol and tobacco, were taken into account.

The fall in the study subjects' IQ showed a direct link to the amount of cannabis they smoked.

But the participants who started smoking cannabis in high school showed the most pronounced effect. Those who continued smoking for years after starting in their teens lost an average of eight points on their IQ scores.

The childhood IQ score was not found to be fully restored, even if they gave up or cut down on cannabis use later in life.

The research team concluded that persistent cannabis use over two decades is associated with "neuropsychological decline." They added that the decline was greater for people who used cannabis persistently.

According to research team member Terrie Moffitt, the findings suggest that cannabis use in adolescence may be toxic to the brain, because it is still undergoing critical development.

She said some members of the study group had become chronic users of cannabis, and that the study had enjoyed a high level of cooperation and participation over the 25-year period, which helped the research team to produce more accurate results.

She said the participants were recruited at the beginning of the study, in 1972, and had stuck with the research project because of confidentiality guarantees.

According to professor Robin Murray of the Institute of Psychiatry at King's College London, who was not involved in the study, there is a mounting body of clinical evidence that cannabis users are generally less successful in education, relationships and careers.

Murray said the study was of high quality, and made him fairly sure that under-18s should not use cannabis, although it might be safe for people who were over 18.

He said the Dunedin participants had been studied very intensively, and that the quality of the results was likely to be high, although further studies would be needed to repeat and validate them.

Murray said the study had found good scientific evidence to support the view that heavy cannabis users gradually lost their talents and failed to achieve their potential.

He said that public education campaigns would be needed to warn people of the risks of cannabis use in young teens, should the results be confirmed by further research.

The study was published in the US journal Proceedings of the National Academy of Sciences.


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