Latest Science Debunks Claim That Marijuana Significantly Harms Brain
[image: A joint sits behind marijuana bud on a white background] 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 emerging science, public opinion and the plant’s rapidly
changing cultural status.Nikka Serrano / EyeEm / Getty Images
By Paul Armentano <>, Truthout
<> Published March 20, 2019
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
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

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

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]

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.”
*Long-Term Exposure*

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
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
*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. *
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Paul Armentano <>

Paul Armentano is the deputy director of the National Organization for the
Reform of Marijuana Laws <>, and is the co-author of the
book *Marijuana Is Safer: So Why Are We Driving People to Drink?*
<> (Chelsea Green,
More by this author… <>