Marijuana use among young adult non-daily cigarette smokers over time.

Recent data regarding growth in concurrent use of nicotine and marijuana have raised concern that reductions in legal restrictions on marijuana use may increase risk for tobacco-related harms. Previous studies have shown cross-sectional links between use of both substances, but less is known about associations over time. The goal of the present study was to test the hypothesis that there is a bidirectional relationship between use of marijuana and use of tobacco products over time, such that increasing use of either substance would predict increasing use of the other. Participants (n = 391, 52% male) were 18-24 year-old Californians who were non-daily cigarette smokers at enrollment and had never been daily smokers. They reported nicotine/tobacco and marijuana use quarterly over 2 years. Longitudinal negative binomial and logistic regression models indicated that each additional timepoint at which participants reported recent marijuana use predicted 9-11% increases in tobacco quantity and frequency. Additionally, each additional timepoint at which cigarette or tobacco use was reported predicted 19-22% greater marijuana frequency. Data suggest that young adults who use marijuana more frequently are likely at risk for greater tobacco exposure, and vice versa. These findings suggest a need for preventive measures that focus on concurrent use of both substances rather than either individually. Published by Elsevier Ltd.