Published in the journal Biological Psychiatry in January 2021, this study examined the effects of alcohol and cannabis on 436 twins, aged 24. It found no brain structural relationship with cannabis use, but a negative brain structural relationship with alcohol.
“Greater alcohol, but not cannabis, misuse was associated with reduced thickness of prefrontal and frontal medial cortices, as well as [the] temporal lobe, intraparietal sulcus, insula, parietal operculum, precuneus, and parietal medial areas.”
The researchers from the University of Minnesota performed a quasi-experimental co-twin control design to separate alcohol or cannabis exposure effects during emerging adulthood from familial risk on control/salience network cortical thickness.
A good summary article and link to the research paper is here.