Tobacco use is the greatest cause of preventable death in the United States and an important health behavior to study among young adults. Prior research has suggested that there is an association between tobacco and marijuana use. Studying these two substances together can provide important insight into patterns of young adult tobacco and marijuana initiation and continuation. This dissertation employed a Sequential Explanatory Mixed Methods design to study tobacco and marijuana co-use among young adults 21-30 years old.

Quantitative data analyses used National Health and Nutrition Examination Survey (NHANES) data to assess prevalence of cigarette and marijuana co-use (Aim 1), and characteristics of co-users (Aim 2) over a 10-year period (2005-2014). Prevalence of past-month cigarette use decreased from 30.9% in 2005-2006 to 23.7% in 2013-2014 (p = 0.024) while both past-month marijuana use (average 18.0%) and past-month co-use (average 9.8%) remained stable between 2005 and 2014. Prevalence of past-month co-use differed significantly by gender (p < 0.001) and racial and ethnic group (p < 0.001). Education level, marital status, race and ethnicity, ratio of income to the poverty level, depressive symptoms, non-marijuana illicit drug use, alcohol use, and household tobacco exposure differentiated co-users from neither users in a multinomial regression. 

Results from analyses of NHANES data, prior literature, and theoretical constructs were used to develop a guide for 20 in-depth interviews with young adult co-users living in the state of Maryland (Aim 3). Interviewees reported two modes of co-use: simultaneous and sequential. Participants reporting using tobacco as a replacement for marijuana in situations where they cannot access or use marijuana, suggesting the two products play similar yet distinct roles in co-use. Influences across levels of the Social Ecological Model were salient in young adults’ co-use. Quantitative results and qualitative findings were interpreted together, and five mixed methods meta-inferences emerged as important in understanding co-use. 

The co-use of tobacco and marijuana is an important behavioral phenomenon to study among young adults. Tobacco and marijuana co-users have unique characteristics compared to tobacco-only and marijuana-only users. Qualitatively, co-users described patterns of product use and replacement that illustrate the complexity of co-use behaviors.