We know from studies of smoking initiation and nicotine dependence that there is significant genetic and environmental covariance between initiation and dependence (1). A number of twin studies have revealed that genetic and environmental effects can explain significant proportions of variance in initiation, abuse and dependence for a variety of licit and illicit substances including cannabis (1–17). Moreover, progression from initial drug involvement, to regular use and abuse appear to be influenced by the some of the same genetic and environmental factors (3, 15, 17).
