Past marijuana use has been found to be significantly related to worse verbal memory in middle age, according to research published in JAMA Internal Medicine.
Impaired cognitive functioning is an acute effect of marijuana use, but according to the authors, “it is unclear whether it has long-term effects on memory and other domains of cognitive function.”
In order to investigate the association between cumulative lifetime exposure to marijuana and cognitive performance in middle life, Stefan Kertesz, MD, an associate professor with the University of Alabama at Birmingham School of Medicine and colleagues used data from the Coronary Artery Risk Development in Young Adults (CARDIA) study, a cohort of 5115 black and white men and women aged 18 to 30 years at baseline from March 25, 1985, to June 7, 1986 (year 0), and followed up over 25 years to August 31, 2011.
The study focused on community-based adults, where casual marijuana use tends to be more common than addiction.
The researchers estimated cumulative years of exposure to marijuana (1 year = 365 days of marijuana use) and assessed associations with cognitive function at year 25.
At year 25, the researchers assessed verbal memory with the Rey Auditory Verbal Learning Test, assessed processing speed with the Digit Symbol Substitution Test, and assessed executive function with the Stroop Interference Test.
The researchers found that among the participants whose cumulative marijuana use exceeded the equivalent of one joint a day for 5 years, there was a significant decline in verbal memory in middle age.
“For every five years of marijuana exposure, one out of two participants would remember one word less,” Dr Kertesz said in a statement.
The researchers also found that in 2012, 37% of students in the 12th grade (ages 17-18 years) had used marijuana in the last year, 23% had used marijuana in the last 30 days, and 6.5% used marijuana daily.
“It’s crucial to recognize that young brains are truly different and not fully developed until age 22 and are at more risk from marijuana,” Dr Kertesz said. “Parents and teachers need to be vigilant that this poses a larger risk to adolescents.”
Dr Kertesz noted that it is also important to realize that marijuana is more potent today than it was in the 1980s, raising the possibility that those who use today’s marijuana may face cognitive consequences greater than those reported in this study.
Auer R, Vittinghoff E, Yaffe K, et al. Association Between Lifetime Marijuana Use and Cognitive Function in Middle Age: The Coronary Artery Risk Development in Young Adults (CARDIA) Study. JAMA Intern Med. 2016; doi:10.1001/jamainternmed.2015.7841.