Both of these examples challenge paradigms about the relationship between addiction and cancer. Epidemiologic data clearly support the association of addiction with cancer: smoking and nicotine dependence are associated with lung cancer; and alcohol consumption and alcohol dependence are associated with esophageal cancer. As a result, exposure to smoking and alcohol has been considered an environmental variable to be controlled in the study of cancer. However, the strongest genetic findings for the development of addiction are also the strongest genetic predictors for the correlated cancers. These findings blur the distinction between genetic and environmental risks with nicotine and alcohol addiction. It also remains unclear if the mechanism of these associations of cancer with the genetic variants can be completely explained through addictive behaviors, or if biologic mechanisms act in the brain to increase the risk of addiction while also acting in the lung and esophagus to increase the risk of cancer. Only through animal models will we be able to separate the genetic influence of these variants on the development of dependence from the genetic contribution to the development of cancer.
