The increased likelihood of co-occurring substance use among early users of a given substance of abuse is well documented. Early age at first drink has been tied to cigarette smoking and illicit drug use in several studies (Callas et al., 2004; Ellickson et al., 2003; Schmid et al., 2007; Vieira et al., 2007). Early onset cigarette smoking has similarly been associated with both alcohol and cannabis use (Korhonen et al., 2008; Kokkevi et al., 2006; Sartor et al., 2008). Prior use of alcohol or cigarettes has, in turn, been found to predict experimentation with cannabis at a young age (Coffey et al., 2000; Ellickson et al., 2004b). Consistency across substances in the course of use has been documented as well. Individuals who begin smoking cigarettes at a young age are also likely to start drinking and using cannabis at a young age (Agrawal et al., 2006). The significant overlap across licit and illicit drugs in patterns of initiation and their associated outcomes suggests the possible role of common vulnerability factors and the utility of examining initiation of use for alcohol, cigarettes, and cannabis in combination.
