Marijuana use does not cause brain abnormalities

By Fred Gardner November 9, 2015

Dr. Dale Deutsch has forwarded an important study from the Journal of Neuroscience showing that marijuana use does not cause structural brain changes.  This is a reversal of the conventional wisdom —a Tashkin-level expose. (UCLA pulmonologist Donald Tashkin and colleagues reported in 2005 that smoking marijuana does not cause lung cancer.) And just as Tashkin’s findings were buried by the Biomedical Establishment and corporate media,  so, too, has the paper by Barbara J. Weiland and colleagues at the University of Colorado and the University of Louisville —published in January— been actively ignored.

Reviewing the papers purporting to show that marijuana use alters brain morphology, Weiland et al note serious inconsistencies—

“Marijuana use has been associated with both increased (Cousijn et al., 2012) and decreased (Yücel al., 2008; Demirakca et al., 2011; Solowij et al., 2011) volumes of subcortical structures, or both (Battistella et al., 2014).”

and a serious limitation:

“Importantly, these studies were not designed to determine causality (i.e., that marijuana use causes morphological changes), which would require a longitudinal design to establish temporal precedence.”

Moreover, the conventional wisdom is based on studies that 

“…did not adequately exclude the effects of confounding variables. Several reports included marijuana groups that differed from control groups in alcohol use/abuse (Demirakca et al., 2011; Solowij et al., 2011; Schacht et al., 2012; Gilman et al., 2014). Unlike marijuana, alcohol abuse has been unequivocally associated with deleterious effects on brain morphology and cognition in both adults (Sullivan, 2007; Harper, 2009) and adolescents (Nagel et al., 2005; Medina et al., 2008; Squeglia et al., 2012). Statistically controlling for comorbid alcohol abuse, as many studies do, is not an ideal strategy, especially in small groups or under conditions where covariates may interact with the independent variable (Miller and Chapman, 2001). Thus, it is possible that alcohol use, or other factors, may explain some of the contradictory findings to date.”

Weiland et al did brain scans on 29 adult daily users and 50 adolescent daily users, and compared them to scans of an equal number of non-users (controlling for age, sex, etc.).

“We evaluated the following structures that were the focus of recent studies of marijuana: the bilateral nucleus accumbens and amygdala (Gilman et al., 2014); hippocampus (Demirakca et al., 2011; Schacht et al., 2012); and cerebellum (Solowij et al., 2011; Cousijn et al., 2012).”

The studies reporting brain damage have all been widely publicized by the mass media and accepted as Scientific Truth.  For example, the New York Times ballyhooed Dr. Jodi Gilman’s findings in an October 29, 2014 article wittily headlined, “This is your brain on drugs.” Reporter Abigail Sullivan Moore visited Gilman at the Harvard Center for Addiction Medicine, and rhapsodized, “The gray matter of the nucleus accumbens, the walnut-shaped pleasure center of the brain, was glowing like a flame, showing a notable increase in density,” as Gilman showed her what addiction looks like on a computer screen. Moore was hooked: “Even in the seven participants who smoked only once or twice a week, there was evidence of structural differences in two significant regions of the brain. The more the subjects smoked, the greater the differences.”

The self-styled “paper of record” ignored the study by Weiland et al, although, as the authors note matter-of-factly:

“The analyses we performed duplicated those previously used (Gilman et al., 2014) with several important differences.

“Our study included more subjects in adult and adolescent samples, and compared extreme groups of non-marijuana users to daily users. Most importantly, the groups were closely matched on an alcohol problem measure (AUDIT) and were not different on many possible confounding variables (e.g., tobacco use, depression, impulsivity, age, and gender). In other words, the present analyses had greater power to detect group differences, while closely controlling for other ef- fects. We found no evidence of differences in volumes of the accumbens, amygdala, hippocampus, or cerebellum between daily versus nonusers, in adults or adolescents. Moreover, effect size data suggest that potential effects are modest and would require very large sample sizes to detect significant differences. The lack of significant differences between marijuana users and control subjects in the present study is consistent with the observation that the mean effect size across previously published studies suggests no clear effect of marijuana on gray matter volumes.”

Dr. Tod Mikuriya once used the term “the new phrenology” to describe a study showing marijuana use causing changes in the brain. And although structural changes would be worrisome, Mikuriya said, it would still have to be shown that those changes result in adverse cognitive or behavioral changes. Weiland et al make the same point:

“It is also unclear how variations in the morphology of cortical or subcortical structures would be interpreted. For example, others have interpreted reductions of gray matter volume in the accumbens as evidence of the deleterious effects of alcohol (Makris et al., 2008), yet increases in accumbens volume associated with marijuana use were interpreted as deleterious (Gilman et al., 2014). Future research should link structural differences to behavioral or functional measures to better understand the implications of differences in brain morphology. In addition, the morphological techniques used for analyses show substantial variation in results depending on processing and software, particularly shape analysis (Gao et al., 2014).”