Long-term Marijuana Use Linked to Poorer Memory: Study

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With the rise of pot being legal in some states in the U.S., there's been a lot of speculation about its effect on the cognitive function of users. Now, a new study recently published in the JAMA Internal Medicine reveals that long-term marijuana could result to poorer memory.

Using data from a study by the Coronary Artery Risk Development in Young Adults (CARDIA), the researchers assessed the participants based on three cognitive domains: Rey Auditory Verbal Learning Test (verbal memory), the Digit Symbol Substitution Test (processing speed), and the Stroop Interference Test (executive function).

The study focused on 3,385 black and white men and women aged 18 and 30 years old during the start of the study in 1985 and 1986. The participants were observed for 25 years. After which, as per CBS News, the participants underwent four tests to measure their cognitive function. They were tested on their ability to memorize from a given list of 15 words, their working memory, attention skill, visual speed and problem-solving skills.

The study in JAMA revealed that out of the 3,385 participants of the study, only 392 or 11.6 percent continued to use marijuana religiously until middle age. These long-term middle-aged marijuana users had a slower pace in processing and poor verbal memory compared to those who stopped or had less pot use. Also, the study noted that being exposed to pot for a long time resulted to the decline of their three cognitive domains.

"We were really surprised by the findings," study author Dr. Reto Auer told CBS News. "The more you smoke, the lower your verbal memory."

The publication also noted that those who used marijuana beyond the 25-year period had a declining verbal memory. Washington Post said that the researchers used a unit of measurement called "marijuana-years," which is equal to an accumulated 365 days of using pot.

CBS News said that through the 15-word memory test, researchers discovered that five years of marijuana use equals to one less word remembered. According to Washington Post, the findings may seem not much, but if you add up the number of marijuana years, the cognitive difference presents a bigger problem.

"Cannabis is a drug, and like all drugs it can harm some users when they use it in particular ways. This study adds to the substantial evidence that the daily use of cannabis over periods of years and decades can harm the mental and physical health of people who use it in this way," said Wayne Hall, director and inaugural chair of the Centre for Youth Substance Abuse Research at the University of Queensland.