Association between Lifetime Marijuana Use and Cognitive Function in Middle Age The Coronary Artery Risk Development in Young Adults (CARDIA) Study

A research study exploring verbal memory loss and exposure to marijuana.

Reto Auer,MD, MAS; Eric Vittinghoff, PhD, MPH; Kristine Yaffe, MD; Arnaud Künzi, BA; Stefan G. Kertesz, MD, MSc; Deborah A. Levine,MD, MPH; Emiliano Albanese,MD, PhD, MPH; Rachel A. Whitmer, PhD; David R. Jacobs Jr, PhD; Stephen Sidney, MD; M. Maria Glymour, ScD, MS; Mark J. Pletcher,MD, MPH

JAMA Internal Medicine. doi:10.1001/jamainternmed.2015.7841 Published online February 1, 2016.

In the February 2016 issue of JAMA Internal Medicine, Auer and colleagues reported an association between cumulative lifetime marijuana exposure and cognitive performance.  This was a prospective study of 3385 middle-aged adults who were followed up for 25 years.  Among the 3385 participants with cognitive function measurements at the year 25 visit, 2852 (84.3%) reported past marijuana use, but only 392 (11.6%) continued to use marijuana into middle age.

The authors concluded that past exposure to marijuana is associated with worse verbal memory, but does not appear to affect other domains of cognitive function.  Their findings suggest that those who used marijuana on a long-term daily basis have poorer verbal memory in middle age than do their peers who have not smoked marijuana habitually.

To go into more detail, after excluding current marijuana users and adjusting for potential confounders, cumulative lifetime exposure to marijuana remained significantly associated with worse verbal memory.  For each 5 years of past exposure, verbal memory was 0.13 standardized units lower, corresponding to an average of 1 of 2 participants remembering 1 word fewer from a list of 15 words for every 5 years of use.  The authors found no associations with lower executive function or processing speed following adjustments for other factors and excluding current marijuana users. 

Some of the limitations of this very ambitious study are the use of self-reporting of marihuana use. In addition, the age of exposure, especially during adolescence and young adulthood, was not queried. The absence of baseline data on cognitive performance makes it difficult to exclude the possibility that those with lower cognitive abilities were more likely to become long-term users of marijuana to begin with.  But despite these limitations, this study did control for core demographic variables, including educational level, race, and sex, which in turn, are associated with baseline cognitive performance.

In summary, participants with the greatest cumulative exposure to marijuana had the poorest verbal memory.  Verbal memory loss corresponded to 1 of every 2 participants remembering 1 word fewer from a list of 15 words for every 5 years of marijuana use.