As is evident from this review, multiple aspects of research into the co-occurring use of cannabis and tobacco require attention. This review, in particular, highlights the importance of genetic influences on various mechanisms that contribute to and might be a consequence of co-occurring use. Despite evidence that cannabis use disorders are heritable, genetic studies have struggled to identify gene variants associated with it – studying it in the context of co-occurring smoking behaviors may facilitate these gene searches. The identification of environmental factors, including the important role of route of administration needs to be carefully examined. Importantly, these environmental factors may further modify genetic susceptibility – this requires study. Also, as shown in Figure 3, these psychoactive substances are linked at multiple stages in their progression of use – in fact, they may be irreparably coupled as early as the emergence of drug expectancies, exposure or initial reactions. Identifying the earliest stage and mechanism for their co-occurrence, following further stages of cross-drug reinforcement and delineating the genetic and environmental underpinnings of these stages will greatly facilitate prevention efforts. Finally, given
