Heavy cannabis use impairs intelligence of young people


A paper presented at the annual congress of the European College of Neuropsychopharmacology (ECNP) in Berlin says that moderate cannabis use by adolescents does not lead to educational or intellectual decline, but that heavy cannabis use is associated with slightly poorer exam results at age 16.

Adolescents who have smoked cannabis 50 times by the age of 15 show ‘impaired’ educational abilities.

The results come from the Avon Longitudinal Study of Parents and Children (ALSPAC, also known as "Children of the 90's") a long-term study that follows the health of children born in the Bristol area (UK) in 1991 and 1992.
The participants each had their IQ tested at age 8 and again at 15, when they also completed a study on cannabis use and other lifestyle factors.

The IQ results were correlated with their GCSE results a year later to see what impact the drug had on exams.

The researchers could find no link between light cannabis use and their exam results, mainly because there was no way to pull apart the various impacts on exam scores.

Negative behaviour such as drinking, smoking and other drug use each had an impact on exams, so there was no method of isolating the impact of light cannabis use.

But they found that the impact of heavy cannabis use was so significant that they were able to pick it out despite the other factors.

Those who had smoked cannabis more than 50 times before they were 15 did distinctly worse in their GCSEs - with an average 3 per cent results drop - even after adjusting for previous educational performance, alcohol, cigarette and other drug use.

Lead researcher Claire Mokrysz, who presented her results at the congress of the European College of Neuropsychopharmacology in Berlin, said: ‘Adolescent cannabis use often goes hand in hand with other drug use, such as alcohol and cigarette smoking, as well as other risky lifestyle choices.

‘It’s hard to know what causes what - do kids do badly at school because they are smoking weed, or do they smoke weed because they’re doing badly? This study suggests it is not as simple as saying cannabis is the problem.’

But she added: ‘The finding that heavier cannabis use is linked to marginally worse educational performance is important to note, warranting further investigation.’

The chair of the congress, Prof Guy Goodwin of the University of Oxford, said: ‘This is a potentially important study because it suggests that the current focus on the alleged harms of cannabis may be obscuring the fact that its use is often correlated with that of other even more freely available drugs and possibly lifestyle factors."

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