This paper estimates the effects of alcohol prices, marijuana decriminalization, cocaine prices, and heroin prices on the demand for these four substances. Both own price effects and cross price effects are estimated. The estimated price elasticities for alcohol, cocaine, and heroin are, respectively, –.30, –.28 and –.94. Marijuana decriminalization was found to increase the probability of marijuana participation by about 8%. The results for the cross price effects provide general evidence of complementarity. It is estimated that decriminalization of cocaine and heroin might lead to about 260,000 new regular cocaine users and about 47,000 new regular heroin users. (JEL 110)