A strong correlation exists between smoking and the use of other substances like alcohol and cannabis. Smoking is positively correlated with alcohol consumption, the severity of alcohol dependence (1) and with the use of cannabis (2). Twin and family studies have shown that smoking behavior (3–5), alcohol consumption (6–8) and cannabis use (9–11) are influenced by genetic factors. Heritability estimates range from low to moderate for initiation of substance use, to rather high for quantity and dependence (3, 5, 11–13). The co-morbidity of tobacco-, alcohol-, and cannabis use is mediated by common genetic influences (14–16).
