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Past marijuana use linked to cognitive impairment

Long-term use associated with worse verbal memory, but not other cognitive deficits

Louise Prime

Tuesday, 02 February 2016

Middle-aged people who were long-term smokers of marijuana were more likely than non-users to suffer from worse verbal memory, research has shown. The authors of the study*, published online today in JAMA Internal Medicine, said other areas of cognitive function appeared to be unaffected; commentators** called for a closer look at how young people view and interpret advice on drug risks, to improve the effectiveness of that advice.

It had not been clear whether or not occasional or low-level use of marijuana earlier in life had any long-term effects; nor whether the degree and duration of impairment was related to intensity of use, or to the age at which drug use began. So researchers used data from the Coronary Artery Risk Development in Young Adults (CARDIA) study – which includes 25 years of repeated measures of marijuana exposure, starting in early adulthood – to see whether or not men and women’s cumulative exposure to the drug was associated with any long-term problems.

In year 25 of the CARDIA study, participants underwent cognitive performance testing, including standardised tests of verbal memory, processing speed and executive function. At this visit, data on cognitive function were available for 3,385 (96.7%) of the 3,499 participants. Of the 3,385, a high proportion (2,852 – 84.3%) said they had used marijuana in the past, and 392 (11.6%) were still using the drug in middle age. This level of exposure was said to be typical of the communities in which participants lived.

The researchers found that past exposure to marijuana was associated with poorer scores in the verbal memory test, but not with worse function in other cognitive areas. They calculated from their results that, for every five years of past exposure to marijuana, around half of participants could remember one word fewer from a list of 15 words.

They acknowledged that their study was limited by the fact that it relied on participants’ self-reported use of marijuana, which might not always provide reliable data. Nevertheless, they concluded: “Future studies with multiple assessments of cognition, brain imaging and other functional outcomes should further explore these associations and their potential clinical and public health implications. In the meantime, with recent changes in legislation and the potential for increasing marijuana use in the United States, continuing to warn potential users about the possible harm from exposure to marijuana seems reasonable.”

The authors of a related commentary called for greater understanding of how young people interpret public health advice on drug use. They said: “The public health challenge is to find effective ways to inform young people who use, or are considering using, marijuana about the cognitive and other risks of long-term daily use. Young adults may be sceptical about advice on the putative adverse health effects of marijuana, which they may see as being overstated to justify the prohibition on its use. More research on how young people interpret evidence of harm from marijuana and other drugs would be useful in designing more effective health advice.”

* Auer R, Vittinghoff E, Yaffe K et al. Association between lifetime marijuana use and cognitive function in middle age: the Coronary Artery Risk Development in Young Adults (CARDIA) study. JAMA Intern Med. Published online February 1, 2016. doi:10.1001/jamainternmed.2015.7841.

** Hall W, Lynskey M. Long-term marijuana use and cognitive impairment in middle age. JAMA Intern Med. Published online February 1, 2016. doi:10.1001/jamainternmed.2015.7850.

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