New Studies on Cannabis Use Yield Some Answers and More Questions

Man smoking marijuana
Man smoking marijuana
Two news studies document various effects of marijuana use on the brain, but many questions remain.

Two recent studies documenting the effects of cannabis use on the brain did not do much for the many questions that remain about the safety and long-term implications of using marijuana.

Published in JAMA Psychiatry, the studies explored cannabis’ effect on subcortical brain structure and brain maturation in adolescence in relation to risk of schizophrenia.

In the first study, David Pagliaccio, PhD, of the National Institutes of Mental Health, and colleagues analyzed the effects of cannabis use in a sample (n=483) of twins and siblings enrolled in the Human Connectome Project, 262 of which who reported using cannabis in their lifetime.

The researchers found that cannabis use was related to smaller left amygdala (approximately 2.3%; P = .007) and right ventral striatum (approximately 3.5%; P < .005) volumes, though differences were within the range of normal variation. The association between amygdala volume and cannabis use was largely owed to shared genetic factors (ρg = −0.43; P = .004), while the relationship between the right ventral striatum and cannabis use was not clear. Brain volumes did not differ between sex-matched siblings with different histories of cannabis use (fixed effect = −7.43; t = −0.93, P = .35), while both exposed and unexposed siblings in pairs with differing cannabis use showed reduced amygdala volumes relative to members of concordant unexposed pairs (fixed effect = 12.56; t = 2.97; P = .003).

The researchers concluded that differences in amygdala volume in those who used cannabis was attributable to common predispositional factors of genetic or environmental origin, with little evidence indications causal relationships. However, causal influences, in isolation or conjunction with predispositional factors, may apply to the ventral striatum or other brain regions. Additionally, the researchers didn’t rule out the possible effects of more severe levels of cannabis use on brain structure and called for further studies to be conducted.

“Future work characterizing the roles of causal and predispositional factors underpinning neural changes at various degrees of cannabis involvement may provide targets for substance abuse policy and prevention programs,” the authors wrote.