Reefer Madness — How Bad Is It?

Steven Dubovsky, MD reviewing Pagliaccio D et al. JAMA Psychiatry 2015 Aug 26. Gage SH et al. Biol Psychiatry 2015 Aug 12. Goldman D. JAMA Psychiatry 2015 Aug 26.

Although evidence for cannabis-associated specific structural change in a meta-analysis is weak, evidence for a link between psychotic symptoms and cannabis use is strong.

Cannabis use has been associated with altered structure and function of the brain, but which comes first? Two recent articles address the question of causality.

Pagliaccio and colleagues obtained genetic data and neuroimaging from 50 monozygotic twin pairs, 45 dizygotic twin pairs, and 146 sibling pairs. The total group included 89 same-sex pairs discordant for self-reported cannabis use, 81 concordant for cannabis use, and 71 concordant for no cannabis use. Cannabis use was associated with smaller left amygdala and right ventral striatum (VS) volumes, although these were within the normal range. Amygdala volumes were similar within pairs, regardless of concordance of cannabis use, which suggests a genetic predisposition. The cause of smaller VS in cannabis users was not clear.

Gage and colleagues comprehensively reviewed published studies and a meta-analysis on effects of cannabis use. Even though the examined studies did not fully control for genetic and other factors that predispose to cannabis use, there was good evidence that chronic cannabis use doubles the risk for developing schizophrenia and further increases risk for other psychoses. Cannabis use posed even higher risk in individuals predisposed to psychotic illness.

Citation(s):

Pagliaccio D et al. Shared predisposition in the association between cannabis use and subcortical brain structure. JAMA Psychiatry 2015 Aug 26; [e-pub].

Gage SH et al. Association between cannabis and psychosis: Epidemiologic evidence. Biol Psychiatry 2015 Aug 12; [e-pub]. 

Goldman D.America's cannabis experiment. JAMA Psychiatry 2015 Aug 26; [e-pub].