Despite drug- and drug-class specific mechanisms of action and psychoactive effects, there is substantial overlap of genetic factors underlying addiction to most classes of drugs. For example, approximately 60% of genetic influences are shared between nicotine and alcohol dependence (7). The notion of a shared biological component underlying addiction to different drugs of abuse is reflected in the high rates of co-morbid dependence to different substances, similar patterns in the initiation and continuation of drug use, evidence of cross-tolerance and cross-dependence to different substances, and common mechanisms underlying drug reward in the brain. While the heritability of drug addictions has been determined by many twin and family studies, our understanding of the specific genes involved remains limited.
