When in the development of cannabis use disorders do genetic influences unfold? Twin studies suggest that a high proportion of the genetic influences on cannabis use disorders overlap with genetic influences on earlier stages of experimentation and regular cannabis use [147-150]. Twin analyses of the extent to which genetic influences on the liability to experiment with cannabis (lifetime use) are also responsible for genetic variation in the liability to cannabis use disorders, indicate that nearly 80−90% of these genetic factors were common to earlier and later stages of cannabis involvement [151]. Presumably, some of the genes described here specifically influence early stages of involvement (e.g. DRD2 and early initiation of cannabis use) while others may contribute across several stages, including abuse/dependence (e.g. CNR1). To this end, genomic studies may need to include a range of controls, for instance, those who have never used cannabis (unexposed) as well as those who have used cannabis a few times but never developed problems (exposed controls). Such a stratified approach to cannabis involvement is vital to clarifying the genetic etiology of cannabis use disorders.
