School-based drug prevention programs target not only the use of illicit drugs such as marijuana but also licit substances such as alcohol and tobacco. These programs thus have the potential of benefiting society not only by reducing the violence and criminal justice costs associated with abuse of alcohol and cigarettes. This opportunity for multiple benefits raises the interesting question, which is most important? Is school-based drug prevention beneficial mainly as a weapon in the war against illegal drugs, or is it more valuable as a promoter of public health through reduction of licit-substance abuse? The authors analyze the costs and effectiveness of several scientifically evaluated drug prevention programs, along with the social costs of the use of various substances. They verify previous findings that the social benefits of drug prevention can exceed its costs, and they conclude that approximately two-thirds of the programs' value to the public stems from reduction in the use of alcohol and tobacco. Implications for program funding are drawn, and the limitations of prevention are discussed. (Contains 92 references and 41 tables.) (Author)