The study was led by researchers from the Prevention Research Center of the Pacific Institute for Research.
Using data from 11th graders who participated in the Student Wellness Survey from 2010-2018, researchers assessed past-30-day co-use changes in counties with low, medium, and high densities of licensed marijuana and alcohol outlets.
-A significant post-legalisation increase in past-30-day co-use in 2016 in counties with the highest retail outlet density.
-Significant post-legalisation increases in perceived risk and parent approval of alcohol and marijuana use.
-Legalisation and greater retail availability of both marijuana and alcohol were positively associated with co-use among teens, and beliefs favourable to alcohol and marijuana use.
"Our results suggest that adolescents living in communities with greater retail availability of recreational marijuana and alcohol may have greater indirect access to these substances through diversion, as it is illegal for them to purchase and use them. So, their primary sources are likely to be social rather than commercial," said lead author Dr Grisel Garcia-Ramirez.