Concurrent use of cannabis and tobacco (i.e., across the lifespan or in a given time period) has become a prevalent phenomenon worldwide. Between 41% and 94% of adults who use cannabis, and half of adults seeking treatment for cannabis use, smoke tobacco at some point in their lives (Agrawal and Lynskey, 2009; Australian Institute of Health and Welfare, 2011; Budney et al., 2000, 2006; Clough et al., 2004; Kadden et al., 2007; Martin et al., 1992; Richter et al., 2005; Rigotti et al., 2000; Tullis et al., 2003). Cannabis use is also high among individuals who smoke tobacco, with 25–52% of tobacco smokers using cannabis (Leatherdale et al., 2006, 2007; SAMHSA, 2005). Several mechanisms may explain the strong relationship of cannabis and tobacco, such as involvement of the endocannabinoid system in addiction to both (Castañe et al., 2005; Le Foll et al., 2008; Maldonado et al., 2006), shared genetic liability underlying the risk for use of both (Agrawal et al., 2010; Chen et al., 2008; Neale et al., 2006; Xian et al., 2008; Young et al., 2006; Anney et al.,
