An international team, including researchers from University of California in San Francisco and University of Lausanne in Switzerland, found that for every five years of marijuana use, one out of two people remembered one word fewer from a list of 15 words.

Researchers studied participants who were enrolled in the Coronary Artery Risk Development in Young Adults (CARDIA) study, which included more than 5,000 adults who initially enrolled in the study between ages 18 and 30. The participants reported if they had used marijuana in the previous month during a series of follow-up visits.

After a 25-year follow-up, they were given a series of cognitive tests that looked at verbal memory, processing speed and executive function.

Long-term marijuana use was associated with worse performance in all three tests, media reported. After the researchers adjusted for other factors (such as use of other substances and depression), they found that only the association between long-term use and verbal memory was statistically significant.

This means the associations between marijuana use and both processing speed and executive function may have been due to chance, researchers said. The more marijuana a person used, the greater are the effects on their verbal memory, they said.

The findings were published in the journal JAMA Internal Medicine.

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