Epidemiological and clinical studies have shown that many people subsequently use multiple drugs after their initiation of one drug (105, 109). Twin studies have demonstrated the presence of shared environmental factors that contribute to substance use. The shared environmental influence has a significant effect on tobacco initiation, alcohol use, and any drug use; however, genetic factors have a higher impact than shared environmental influences on tobacco use, tobacco problem use, and marijuana initiation (109). Studies on the etiology of the comorbidity of multiple substances in adolescents have suggested that genetic and environmental influences are common across substance classes (74, 109). In family studies involving adults, findings regarding general versus substance-specific familial risks have not been conclusive (15, 91, 128).
