youth since 1998 (7). Despite cross-national variations in rates of use, cannabis smoking has been and continues to be associated with adverse effects on school, employment, relationships, cognitive processing, mental health, health (primarily respiratory), and health-care utilization (8). Independently, each of these psychoactive substances is associated, to varying degrees, with considerable morbidity (9;10). However, given these associated harms, what is perhaps most alarming is that the use of cannabis and tobacco (primarily as tobacco cigarettes) frequently co-occurs across the lifespan (11;12). In this review, we discuss lifetime co-occurrence, which indicates the use and misuse of cannabis and tobacco at any point (and not necessarily at the same time). Discussion of the implications of simultaneous use (i.e. used on the same occasion) and co-administration (e.g. mulling – adding tobacco to cannabis joints, or blunts – rolling cannabis in cigar paper) is also presented. The extent of and mechanisms underlying the co-occurring use of these psychoactive substances, the consequences, and the treatment implications remain research questions of considerable interest. Figure 1 provides an overview of these various mechanisms - we discuss (a) the epidemiology of lifetime co-occurrence of cannabis and tobacco use; (b) the mechanisms underlying lifetime co-occurring use, including shared genetic
