Cannabis Use Not Associated With Later IQ Decline

Lifetime cannabis use is not associated with cognitive decline, according to longitudinal data published in the journal Brain and Behavior.

Danish investigators assessed the relationship between cannabis use and age-related cognitive decline in a cohort of 5,162 men. Subjects’ IQs were assessed during early adulthood (mean age: 22) and then again in late midlife (mean age: 62).

Researchers determined that participants with a history of cannabis use experienced “significantly less cognitive decline” over their lifetimes than did non-users. Among cannabis consumers, neither age of initiation nor frequency of use was associated with negative effects on cognition.

They reported: “In this study of 5162 Danish men, the mean cognitive decline was found to be 6.2 IQ points over an average of 44 years. Notably, cannabis users exhibited statistically significantly less cognitive decline compared to nonusers … and the association remained significant when controlling for potential confounders. … In the fully adjusted model, cannabis use was associated with 1.3 IQ points less cognitive decline than the decline observed in the reference group.”

The study’s authors concluded: “[These findings] align with most existing studies, suggesting no association between cannabis use and greater cognitive decline. … Among cannabis users, no significant associations with age-related cognitive decline could be demonstrated for age of initiation of cannabis use. Years of frequent cannabis use were generally associated with no significant difference in cognitive decline when compared with no frequent use. … Further studies are needed to investigate whether these findings reflect that there are no adverse effects on cognitive decline or that the effects of cannabis are temporary and disappear after a prolonged period of time.”

Commenting on the findings, NORML’s Deputy Director Paul Armentano said: “These results contradict one of the more prominent and longstanding stereotypes about cannabis and cannabis consumers. It is unfortunate that these stereotypes often go unchallenged in the media and elsewhere. It is even more unfortunate that studies refuting these long-held stereotypes seldom receive the type of mainstream attention they deserve.”

Despite longstanding allegations that marijuana use negatively impacts IQ, few longitudinal studies support this claim. For example, a British study of more than 2,000 teens determined that cannabis exposure prior to age 15 “did not predict either lower teenage IQ scores or poorer educational performance … once adjustment is made for potential confounders.” Several studies involving adolescent twins have similarly failed to report any causal effect of cannabis use on either cognition or IQ. More recently, a literature review published in JAMA Psychiatry concluded: “Associations between cannabis use and cognitive functioning in cross-sectional studies of adolescents and young adults are small and may be of questionable clinical importance for most individuals. Furthermore, abstinence of longer than 72 hours diminishes cognitive deficits associated with cannabis use.”

While a widely reported 2012 study suggested that the onset of cannabis use in early adolescence is associated with lower IQ at mid-life, the authors of that paper were subsequently criticized for failing to properly account for socioeconomic confounders. A response to the study, published in the Proceedings of the National Academy of Sciences, stated: “The methodology is flawed and the causal inference drawn from the results premature. … The causal effects estimated [by the study’s authors] are likely to be overestimates, and that the true effect [of cannabis on lifetime IQ] could be zero.”

Other longitudinal studies assessing the relationship between lifetime marijuana use and IQ have also failed to report significant adverse effects on cognitive performance. Data published earlier this year in the Journal of the American Medical Association similarly reported that adult patients who regularly consume medical cannabis do not experience any significant adverse changes in either brain morphology or cognition.  

The full text of the study, “Cannabis use and age-related changes in cognitive function from early adulthood to late midlife in 5,162 Danish men,” appears in the journal Brain and Behavior. Additional information is available from the NORML Fact Sheet, “Marijuana Exposure and Cognitive Performance.”