Tobacco and marijuana use among adolescents and young adults: a systematic review of their co-use.

Tobacco (TOB) and marijuana (MJ) are the most widely used drugs among adolescents and young adults. The literature on their co-use, however, has not been systematically reviewed. We identified 163 English language articles published from 1999 to 2009 examining TOB and MJ co-use, correlates or consequences of co-use, or interventions for prevention or cessation of co-use with participants aging 13-25 years. Most studies (n=114, 70%) examined TOB and MJ co-use, and 85% of relationships studied indicated a significant association. Fifty-nine studies (36%) examined correlates or consequences of co-use. Factors consistently associated with increased likelihood of co-use, defined as significant associations in at least four studies, were African-American ethnicity, mental and physical health characteristics (e.g., high-intensity pleasure temperament), and school characteristics (e.g., good grades). The only consistent consequence of co-use was exacerbation of mental health symptoms. Few studies examined prevention (n=3) or cessation (n=2) interventions for TOB and MJ co-use, and the findings were stronger for prevention efforts. A sufficient literature base has documented that TOB and MJ use are strongly related in young people, yet few consistent correlates and consequences of co-use have been identified to inform intervention targets.