Daily Marijuana Will Not Shrink Your Brain

During the past few years a pair of sensational studies have been widely featured in the national press; one suggested that daily marijuana use might decrease IQ (Meier et al., PNAS, 2012), the other suggested that daily recreational use caused shrinkage of brain areas that are critical for learning, memory and emotional control (Gilman et al., 2014, J Neurosci).  The report by Meier et al. was immediately challenged (Rogeberg, 2013, PNAS) for failing to take into account the confounding effect of socioeconomic status, a factor which has been shown to have significant impact on IQ score.
The results of the second study have now been confronted by a publication this month (Weiland et al., 2015, J Neurosci) that clearly demonstrated that daily use of marijuana produced no significant changes in the size or shape of brain regions involved in the control of emotion or learning and memory.
This recent study investigated males and females who were either adolescents (ages 14-18, N=262) or adults (ages 18-53, N=503).  Most importantly, the subjects were matched for other lifestyle factors that might have influenced the outcome, such as daily consumption of alcohol or tobacco, ethnicity, depression and anxiety scores and, most importantly, for their degree of impulsive sensation-seeking behaviors.  Many of these critical factors were not controlled for in the two previously published studies.  For example, alcohol abuse has been indisputably associated with harmful effects on brain health and cognitive function.
Thus, overall, this most recent study that included more subjects in both adult and adolescent age ranges, and that were closely matched for alcohol use, tobacco use, depression, impulsivity, age and gender, had greater statistical power to detect any important differences due to daily marijuana use, and found no evidence for any negative effects.
Is this the end of the story? No. Just because no changes were discovered in the morphology of these critical brain regions does not settle the issue of whether daily marijuana use is harmful to the brain.  First, no one fully understands what it means to find variations in the morphology of specific brain regions.  This type of investigation is quite difficult interpret properly and often involves assumptions that are as primitive as 18th century phrenology.  Future studies will also need to consider the various chemical components of marijuana and their individual effects upon the brain, particularly since so many different strains of the cannabis plant are being produced today.
One last great unknown: will the popular press, the public and our policy makers ignore this far less sensational finding that does not vilify the use of marijuana?