Abstract: "As marijuana is legalized in more states, modes of administration that facilitate co-use with tobacco are growing in popularity among young adults. This study examines the prevalence, patterns, correlates, and reasons for co-use so that targeted interventions can be developed to prevent negative consequences associated with tobacco use and co-use...In Fall 2019, 1887 young adults, originally recruited in 2010 from 11 colleges in North Carolina and Virginia to participate in a cohort study, completed an online survey. Co-use was defined as self-reported use of marijuana and tobacco in the past month. Tobacco-only, marijuana-only and co-users were compared using regression modeling... Overall, 9.3% of the sample were co-users, 7.1% tobacco-only, and 15.8% marijuana-only users. Tobacco use was associated with an increased likelihood of marijuana use and vice-versa. Co-users were more likely to use e-cigarettes and blunts to administer marijuana and less likely to use smokeless tobacco products. They were more likely to use cocaine, have less anxiety, and be heavier marijuana users than marijuana-only users. Co-users of e-cigarettes and marijuana were less likely to be daily e-cigarette users and make quit attempts than e-cigarette users that did not use marijuana. Experimentation was the primary reason for co-use of tobacco and marijuana...Co-users were more likely to use modes of administration that facilitate use of both substances and have patterns of use that may impact cessation efforts. These findings highlight the importance of surveillance of co-use and the development of interventions targeting experimentation with these substances by young adults".