What's That Word? Marijuana May Affect Verbal Memory
Credit: Stanimir G.Stoev | Shutterstock.com

Years of smoking pot may have an effect on a person's verbal memory, which is the ability to remember certain words, a new study finds.

For every five years of marijuana use, researchers found that, on average, one out of two people remembered one word fewer from a list of 15 words, according to the study.

Long-term use was not, however, significantly associated with decreases in other measures of cognitive function, such as processing speed or executive function, the researchers wrote in the study, published today (Feb. 1) in the journal JAMA Internal Medicine. Executive function includes skills such as planning and focusing. 

To examine the effects of long-term marijuana use, the researchers studied participants who were enrolled in the long-running Coronary Artery Risk Development in Young Adults (CARDIA) study. The CARDIA study included more than 5,000 adults who initially enrolled in the study between ages 18 and 30. During a series of follow-up visits, the participants reported if they had used marijuana in the previous month. At the 25-year follow-up, the participants were given a series of cognitive tests that looked at verbal memory, processing speed and executive function. [11 Odd Facts About Marijuana]

While long-term marijuana use was associated with worse performance in all three tests, 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 (meaning the associations between marijuana use and both processing speed and executive function may have been due to chance).

"We found a dose-dependent independent association between cumulative lifetime exposure to marijuana and worsening verbal memory in middle age," the researchers wrote in the study. In other words, the more marijuana a person used, the greater the effects on verbal memory.

The researchers are not sure how marijuana exposure might affect verbal memory. One potential explanation is that THC, or tetrahydrocannabinol — the chemical responsible for most of marijuana's psychological effects — may affect the way information is processed in the hippocampus, the researchers wrote in their study.

The researchers noted some limitations to the study, including that the marijuana use was self-reported and that the cognitive tests were only done once, at the end of the study.

Because there were no baseline data available on cognitive abilities, it's more difficult to rule out the possibility that people with lower cognitive abilities were more likely to use marijuana, an editorial accompanying the study added.

Still, the findings fit well with other evidence on long-term use of marijuana and a decline in cognitive abilities, according to the editorial.

For example, a long-running study in New Zealand found that adults who were longtime users of marijuana showed a larger decline in IQ scores compared with those who had never used marijuana or those who had stopped, the authors of the editorial wrote.  (A study from earlier this year, however, found that marijuana use was not associated with a drop in IQ in teens).

So, while it is not yet possible to prove cause and effect between marijuana use and a decline in cognitive abilities, both the researchers and the authors of the editorial agree that it is prudent to continue to warn potential users about the possible harms associated with the drug.

Follow Sara G. Miller on Twitter @SaraGMiller. Follow Live Science @livescience, Facebook & Google+. Originally published on Live Science.