While the effects of alcohol use independently on the brain are better understood, the effects of marijuana use remain less clear. Some studies have found evidence of structural and functional brain abnormalities [5,6,20], while other neuroimaging studies report no differences in the brains of heavy marijuana users compared to non-using controls [15,22,29,39,49,100,108]. In previous studies, our group has found between-group differences in cognition and brain activity in heavy marijuana using adolescents compared to controls [67,81,94]. In this sample, white matter tracts in adolescents who binge drink and use marijuana were more coherent than in adolescents reporting only binge drinking. These findings are unexpected, as recent animal studies have found that rats receiving the psychoactive principal component of marijuana (THC) along with alcohol show an enhanced susceptibility of the developing brain to incur alcohol-related apoptotic neuronal cell death [40].
