Taking into account this heterogeneity of the clinical phenotype, it may seem surprising that the risks for addictions related to the specific drugs are highly correlated. Considering that these risks have substantial (moderate to high) heritability, i.e., genetic contribution to phenotypic variance, it is indeed noteworthy that this contribution is largely non-specific, at least for addictions to illicit drugs (Kendler et al., 2003). Therefore, variation in the substance-specific addiction risks is largely due to shared biological mechanisms, leading to compulsive drug seeking, a defining feature of addiction (Conway et al., 2010; Koob and Volkow, 2010). This commonality is thus unlikely to derive from drug metabolism.
