The co-occurrence of alcohol and drug addiction in the same individuals is highly prevalent in clinical populations. Neurochemical mechanisms constitute the basis for addiction to both alcohol and drugs, namely in the mesolimbic pathway and the locus ceruleus in the brain. Treatment strategies are directed at both alcohol and multiple drug addictions in patient populations. The formulation of a generalized vulnerability that extends to both alcohol and drug addiction is not only possible but also necessary to explain the significant number of individuals who develop both alcohol and drug addictions. Future research that is pertinent and relevant may depend on an adequate understanding of the generalized vulnerability to develop alcohol and drug addictions, diagnostic strategies, and treatment models. 54 references and 1 figure