In an Oregon based study, researchers examined the recreational marijuana legalisation and alcohol retail outlet density levels that are associated with co-use and beliefs supportive of the use of each among teens. 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. Findings include that a significant post-legalisation increase in past-30-day co-use in 2016 in counties with the highest retail outlet density. It also revealed that 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.