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., 2007; Agrawal et al., 2008), similar environmental and/or temperamental influences (Brook et al., 2010; Creemers et al., 2009; Golub et al., 2005; Kelly, 2005), similar behavioral cues due to common route of administration (Agrawal and Lynskey, 2009), and enhanced subjective responses to cannabis via tobacco smoking (Penetar et al., 2005; cf. Cooper and Haney, 2009).
