An opinion article recently published in the Journal of Policy Analysis and Management discusses the legalization of recreational marijuana in United States. One of the key unknowns in the debate over legalization concerns the relationship between alcohol and marijuana use. Researchers have attempted to produce causal estimates of this relationship by exploiting cross-sectional policy and price variation. Because the social costs associated with the consumption of alcohol clearly outweigh those associated with the consumption of marijuana, legalizing the recreational use of marijuana is likely to improve public health, although plenty of unanswered questions remain. Based on existing empirical evidence, the authors expect that the legalization of recreational marijuana in Colorado and Washington will lead to increased marijuana consumption coupled with decreased alcohol consumption. As a consequence, these states will experience a reduction in the social harms resulting from alcohol use. While it is more than likely that marijuana produced by state-sanctioned growers will end up in the hands of minors, we predict that overall youth consumption will remain stable.