Claims that cannabis use is associated with lower cognitive functioning are largely based upon the findings of a single longitudinal study. The paper, published by Madeline Meier and a team of Duke University researchers in 2012, reported that the onset of cannabis use in early adolescence was associated with an average decline of eight IQ points by middle-age.
However, a critique of Meier’s study, published in the Proceedings of the National Academy of Sciences, opined that the reported differences in IQ were consistent with socioeconomic differences among the study’s participants and likely were not attributable to marijuana use. (This criticism is hardly surprising as data has historically shown that those of greater economic means tend to test higher on IQ tests than those who are poorer, and critics have also raised questions as to whether the test itself may possess inherent racial biases.) It countered that the Duke team likely “overestimate[d]” the impact of marijuana on IQ and opined that the “true effect [of cannabis exposure] could be zero.”
This criticism is given additional merit by the fact that several later and better controlled studies have failed to replicate Meier’s initial findings. For example, a British study of more than 2,000 teens reported that cannabis exposure prior to the age of 15 “did not predict either lower teenage IQ scores or poorer educational performance … once adjustment is made for potential confounds.”
Researchers at the University of Southern California, Los Angeles, and at the University of Minnesota similarly assessed the potential relationship between cannabis and IQ in two longitudinal investigations of adolescent twins. They reported: “We find little evidence to suggest that adolescent marijuana use has a direct effect on intellectual decline…. The lack of a dose–response relationship, and an absence of meaningful differences between discordant siblings lead us to conclude that the deficits observed in marijuana users are attributable to confounding factors that influence both substance initiation and IQ rather than a neurotoxic effect of marijuana.”
In fact, even Meier appears to concede this point in her later work. Writing in the journal Addiction in 2018, she and her colleagues acknowledged: “Short-term cannabis use in adolescence does not appear to cause IQ decline or impair executive functions, even when cannabis use reaches the level of dependence. Family background factors explain why adolescent cannabis users perform worse on IQ and executive function tests.”
Most recently, a new study published in The Lancet being widely publicized this week claims that an estimated 30 to 50 percent of psychosis cases in Europe are due to cannabis exposure. However, this study does not establish causation, nor could it because of the observational nature of its design. In fact, the authors clearly state in the abstract that they are “assuming causality.” Given the fact that such a cause-and-effect relationship is yet unproven and there is no consensus among experts that such causation exists, this would appear to be a troubling leap for the authors to take and it should not go unnoticed. Moreover, it is well established that those with psychiatric illness typically use all intoxicants at greater rates than the general public, so the fact that those admitted to institutions for first-episode psychosis are more likely to consume cannabis than are those in the general population is hardly surprising. But it is not evidence that marijuana in any way causes the condition. Finally, the fact that cannabis has been used by various populations for decades at disparate rates, yet rates of psychosis and other psychiatric disorders have generally remained static over this same period of time, strongly argues against a direct causal relationship.
The Developing Brain
While the use of alcohol has been unequivocally associated with deleterious effects on brain morphology in both adults and adolescents, claims that cannabis exposure significantly damages the developing brain are far less persuasive.
Specifically, a team of University of Colorado investigators in 2017 assessed the impact of marijuana exposure and alcohol ingestion on brain structure in groups of adults and adolescents. They reported that, “Alcohol use severity is associated with widespread lower grey matter volume and white matter integrity in adults, and with lower grey matter volume in adolescents.” (Changes in white matter are associated with the onset of certain neurodegenerative diseases while grey matter contains most of the brain’s neurons.) By contrast, “no associations were observed between structural measures and past 30-day cannabis use in adults or adolescents.”
In 2018, researchers from the University of Pennsylvania, Perlman School of Medicine performed a systematic review of 69 studies (published between the years 1973 and 2017) assessing the relationship between adolescent cannabis use and cognition. Researchers reported no significant long-term deficits in memory, attention or other aspects of cognitive functioning that could be independently attributed to cannabis use, regardless of subjects’ age of initiation.
They concluded that, “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…. [R]esults indicate that previous studies of cannabis in youth may have overstated the magnitude and persistence of cognitive deficits associated with [marijuana] use.”
Most recently, University of Pennsylvania researchers compared brain scans of adolescents who used cannabis occasionally (one to two times per week) and frequently (more than three times per week) versus matched controls (similarly aged teens who abstained from the plant). They concluded, “There were no significant differences by cannabis group in global or regional brain volumes, cortical thickness, or gray matter density, and no significant group by age interactions were found…. In sum, structural brain metrics were largely similar among adolescent and young adult cannabis users and non-users.”
While some have theorized that marijuana exposure over the long term may be associated with an adverse impact on the brain, clinical assessments of frequent cannabis consumers challenge concerns. For instance, cumulative lifetime cannabis exposure is not associated with deficits in cognitive performance following drug abstinence.
Long-term exposure is also not associated with changes in working memory. In a 2015 Canadian clinical trial assessing the safety of daily cannabis consumption, users demonstrated no identifiable differences in neurocognitive skills compared to non-using controls. These findings were similar to those of a U.S. trial assessing the health of four Compassionate IND patients (federally authorized medical marijuana consumers) who had smoked cannabis daily for a period of up to 27 years. Authors concluded that, “No consistent or attributable neuropsychological or neurological deterioration has been observed.”
Most recently, University of Colorado scientists compared magnetic resonance imaging scans in 28 cannabis users over the age of 60 versus matched controls. Cannabis-consuming participants in the study, on average, had used marijuana weekly for 24 years. The authors reported that long-term cannabis exposure “does not have a widespread impact on overall cortical volumes while controlling for age” despite over two decades of regular cannabis use on average. This is in contrast to the large, widespread effects of alcohol on cortical volumes that might be expected to negatively impact cognitive performance.
Facts, Not Fears, Should Guide Marijuana Policy
An estimated one out of five Americans now resides in a jurisdiction where the adult use of cannabis is legally regulated, while more than 30 states have enacted statutes authorizing its medical access. Public support for marijuana policy reform has also steadily increased over the past decade, and now two out of three Americans endorse cannabis legalization. It is time for politicians to reject the unsubstantiated “reefer rhetoric” and fear-mongering of the past and move forward to amend U.S. cannabis laws in response to the emerging science, public opinion and the plant’s rapidly changing cultural status.
Editor’s note: This article was updated on March 20, 2019, to address a study that estimates 30-50 percent of psychosis cases in Europe are due to cannabis exposure.