Additionally, a number of twin studies find that underlying all forms of substance use behavior is a shared genetic propensity to use and misuse drugs. Examining this hypothesis, Young and colleagues (32) reported that while the genetic overlap across alcohol, tobacco and cannabis use was modest (r=0.14-0.31), shared genetic influences played a significantly more prominent role when examining alcohol, tobacco and cannabis dependence symptoms (r=0.56-0.62). Expanding this to include other substances, Kendler and colleagues (33) found that an overwhelming 83% of the genetic variance in cannabis abuse/dependence symptoms was shared with genetic influences on alcohol, nicotine, caffeine and cocaine abuse/dependence. However for nicotine dependence, these shared factors were responsible for only 37% of the genetic variance. Thus, while there is consistent support for common genetic factors influencing use and misuse of multiple substances, the extent to which these general factors impact the co-occurring use and misuse of cannabis and tobacco remains unclear.
