Long-Term Marijuana Use Impacts Verbal Memory

Analysis of data from the Coronary Artery Risk Development in Young Adults (CARDIA) study links past marijuana use to poor verbal memory. The results were published in JAMA Internal Medicine.

Evidence points to cognitive impairment with heavy, long-term marijuana use; however, there is limited data on the long term impact of occasional and low-intensity marijuana use early in life.

Using data from the CARDIA study, Reto Auer, MD, MAS, of the Department of Ambulatory Care and Community Medicine at the University of Lausanne in Switzerland, and colleagues aimed to examine the long-term impact of marijuana use.

The CARDIA study included a cohort of 5115 men and women aged 18 to 30 years who were assessed at baseline and then followed for 25 years. Participants underwent repeated assessments for marijuana use and cognitive function was assessed at the end of the study.

In total, cognitive function was measured in 3385 participants, of whom 84.3% (N=2852) reported past marijuana use. Continued use of marijuana was reported in 11.6% (N=392) with current use associated with worse processing speed and verbal memory.

Further, lifetime exposure of marijuana was associated with poorer performance in verbal memory, processing speed, and executive function.

After full adjustment and excluding current use, there was a significant association between lifetime use and verbal memory. The researchers found that for every 5 years of marijuana exposure, there was an associated 0.13 reduction in verbal memory (RAVLT) score (95% CI: -0.24 to -0.02, P=0.02). The authors note that the reduction in verbal memory scores correspond to, “a mean of 1 of 2 participants remembering 1 word fewer from a list of 15 words for every 5 years of use.”

The authors hypothesized that the effects of marijuana may be related to the impact of tetrahydrocannabinol on hippocampal processing of information.

In an accompanying editorial, Wayne Hall, PhD and Michael Lynskey, PhD, of the National Addiction Centre at Kings College London, note that there is no definitive evidence that heavy marijuana use causes verbal memory impairment. However, they write, “Individual studies can always be criticized, but we think considerable weight should be given to the confluence of findings from case-control, neuroimaging, and now 2 long-term, prospective, epidemiologic studies.”

They further point out that regular marijuana use starting at an early age has been associated with dependence which has been tied to poorer psychosocial outcomes.  They suggest that implementing labels with adverse health effects should be considered by state and federal regulators in states with legalized marijuana.

References:

1. 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.
2. Hall W and Lynskey M. Long-term Marijuana Use and Cognitive Impairment in Middle Age. JAMA Intern Med. 2016; doi:10.1001/jamainternmed.2015.7850.