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Frequent Cannabis Use May Trigger Small, Fleeting Cognitive Deficits in Young People

April 24, 2018

By Marilynn Larkin

NEW YORK—Frequent cannabis use by adolescents and young adults is associated with small reductions in cognitive function that diminish after more than three days of abstinence, researchers say.

Dr. J. Cobb Scott of the Hospital of the University of Pennsylvania in Philadelphia and colleagues examined 69 studies including 2,152 cannabis users (mean age, 20.6 years; 68.4% male) and 6,575 comparison participants (mean age 20.8; 55.8% male).

As reported online April 18 in JAMA Psychiatry, analyses of neurocognitive test results (including domains of attention, learning, delayed memory, speed of information processing, executive function, and others) indicated a small overall effect size for reduced cognitive functioning associated with frequent or heavy cannabis use (P < 0.001).

Subgroup analyses showed no significant differences in effect sizes by age category (adolescents or adults), early vs. late onset of cannabis use (ranging from 15 to 18 years old, depending on the study), or whether studies matched groups by alcohol use or period of publication.

Mean age, mean age at first use, and between-group differences in depression were not associated with variability in effect size estimates.

However, studies requiring an abstinence period longer than 72 hours (15 studies; 928 participants) prior to neurocognitive testing had an overall effect size that was not significantly different from 0 and was smaller than in studies with less stringent abstinence criteria (54 studies; 7,799 participants; P = 0.01).

"Associations between cannabis use and cognitive functioning in cross-sectional studies of adolescents and young adults are small and may be of questionable clinical importance for most individuals," the authors state.

"Furthermore," they conclude, "abstinence of longer than 72 hours diminishes cognitive deficits associated with cannabis use."

Dr. Cobb told Reuters Health, "Caution is always warranted when extracting from group data to individual patients, but clinicians may want to question whether larger deficits in cognitive functioning can be attributed to cannabis use."

"For cannabis users concerned about cognitive functioning problems, such as memory problems, abstinence might provide recovery," he said by email.

"We were not able to examine longitudinal studies that follow cannabis users over time," he noted. "But I am somewhat encouraged, as I think the results support findings from longitudinal studies showing that abstinence from cannabis results in a reduction of any cognitive deficits, at least in those who regularly use cannabis for shorter time frames."

"However," he added, "we still have a lot to learn about effects of heavy cannabis use on brain functioning over longer periods of time."

Dr. Margaret Haney, Professor of Neurobiology (in Psychiatry) at Columbia University Irving Medical Center in New York City, told Reuters Health, "I think this is a significant contribution to the literature."

"There is a small but significant association between frequent cannabis use and cognitive deficits in adolescents and young adults," she said by email. "Yet whether these deficits persist after a period of abstinence is a question of considerable public health significance."

"This meta-analysis shows that the association between cannabis use and cognitive deficits among young people largely disappeared when studies required abstinence from cannabis prior to neurocognitive testing," she said.

"Without this control, it is possible that people were either acutely intoxicated or had been quite recently when undergoing neurocognitive tests - a considerable confound when considering the issue of persistent cognitive deficits," she noted.

"Cannabis abstinence was not even confirmed objectively in these studies (this is difficult to do in heavy cannabis users), yet still the relationship largely disappeared," she said.

"No one is arguing cannabis is good for cognition, but this analysis contributes significantly to the discussion of long-term consequences of adolescent cannabis use," Dr. Haney concluded.


JAMA Psychiatry 2018.

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