Individuals with long-term exposure to marijuana may experience significant impairment of verbal memory by the time they reach middle age, warn researchers, who found that other aspects of cognitive function were unaffected.

Investigators, led by Reto Auer, MD, of the Department of Ambulatory Care and Community Medicine, University of Lausanne, Switzerland, found that for each additional 5 years of exposure to marijuana, there was a significant decline in performance on a standard verbal memory test, although the researchers note that the clinical meaningfulness of the findings is open to question.

“Future studies with multiple assessments of cognition, brain imaging, and other functional outcomes should further explore these associations and they’re potential clinical and public health implications,” they write.

“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 research was published online February 1 in JAMA Internal Medicine.

Worse Verbal Memory, Processing Speed The investigators examined data from the Coronary Artery Risk Development in Young Adults study on 5115 black and white men and women from four sites in the continental United States who were recruited between 1985 and 1986 and followed through August 2011.

Current and lifetime marijuana use were assessed at baseline and after 2, 5, 7, 10, 15, 20, and 25 years of follow-up. Cognitive function was assessed at the 25-year follow-up visit using the Rey Auditory Verbal Learning Test, the Digit Symbol Substitution Test, and the Stroop Interference Test.

Linear regression took into account demographic and cardiovascular risk factors, as well as tobacco smoking, alcohol and illicit drug use, physical activity, depression, and the results of the mirror star tracing test, which is a measure of cognitive function, at the 2-year assessment.

Cognitive function test results were available for 3385 participants, of whom 2852 reported lifetime marijuana use. Only 392 individuals were current users; 311 had more than 5 years of cumulative exposure to the drug, and 65 reported daily use. The researchers found that current marijuana use was associated with worse verbal memory and processing speed. Cumulative exposure was associated with worse performance on all three cognitive tests.

Taking into account current users and potential confounding factors, there was a significant association between cumulative marijuana use and verbal memory, such that each additional 5 years of exposure was associated with 0.13 lower standardized units on the RAVLT. The findings were unaffected by adjusting for potential diagnoses of schizophrenia or the use of antidepressant medications.

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