Past heavy marijuana use impacts verbal memory

By Bianca Nogrady
Long ­term past marijuana use is associated with worse verbal memory in middle age, a cohort study has found. 

Analysis of data from a 25 ­year US cohort study in 5115 individuals aged 18 to 30 years at baseline showed that among participants who reported past marijuana use, each five years of exposure was associated with remembering one fewer word from a list of 15 words. 

The study, published in JAMA Internal Medicine, found no impact of past marijuana use on executive function or processing speed. 

‘Impaired cognitive function is an acute effect of marijuana use, and there is increasing evidence that such effects may persist later in life,’ the study authors wrote. ‘Heavy, long­ term use of marijuana has been associated with cognitive impairment, particularly in learning and remembering new information.’ 

Dr Peter Gates, Senior Researcher at the National Cannabis Prevention and Information Centre, said that while confirming a long ­term impact of marijuana use on verbal memory, the study was unique in distinguishing between past and current users. 

‘Most studies looking at cognition are looking at acute effects in people who have just taken cannabis and what it does to the brain,’ Dr Gates told Medicine Today. ‘This study was good in that it was able to look at cognition among those who had used in the past but weren’t currently using.’ 

Commenting on the finding of an effect only on verbal memory, Dr Gates said neuro­imaging studies showed most of the effects of marijuana on the brain were in the areas of the brain associated with verbal learning and working memory. He also pointed out that the method of assessing marijuana use relied on a five-yearly self­-report of the previous month’s use, with the assumption that that single month represented monthly use across the entire previous five years.
JAMA Intern Med 2016; doi:10.1001/jamainternmed.2015.7841. 

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