This paper analyzes the impact of marijuana legalization in Colorado on alcohol consumption. The empirical analysis is based on a sample of adults ages 18 - 39 from Colorado, Utah, Arizona, New Mexico, Wyoming, Nebraska, and Kansas over the years 2008 - 2013. Using data from the Center for Disease Control’s Behavior Risk Surveillance Survey, this research finds that there is no statistically significant effect of the legalization of marijuana on alcohol consumption. Therefore, this study can neither confirm nor refute the hypothesis that the two goods are substitutes or complements. Furthermore, this research finds that marijuana legalization has no significant effect on smoking or BMI.