Alcoholism and drug dependence are common, chronic disorders with considerable personal and societal costs (1). The 12 month prevalence for substance use disorders (abuse plus dependence) in the United States is: alcohol 8.5%, cannabis 1.5%, opioids 0.4% and cocaine 0.3% (2,3). Alcoholism comorbidity in drug dependent individuals is high: 90% for cocaine, 74% for opioids and 68% for cannabis whereas only 13% of current alcoholics have a current drug use disorder (3). It has been shown that one common genetic factor has a strong influence on the risk for dependence on opiates, cocaine, cannabis and other illicit drugs and that most of the genetic and shared environmental risk factors are non-specific (4,5). Therefore there may be substantial shared vulnerability to addiction among substance dependent individuals although there is also evidence for substance-specific transmission factors (6).
