This paper estimates the effect of alcohol use on consumption of hard drugs using the exogenous decrease in the cost of accessing alcohol that occurs when individuals reach the minimum legal drinking age. By using a regression discontinuity design and the National Longitudinal Study of Youth 1997, I find that all measures of alcohol consumption, even alcohol initiation increase discontinuously at age 21 years. I also find evidence that consumption of hard drugs decreased by 1.5 to 2 percentage points and the probability of initiating the use of hard drugs decreased by 1 percentage point at the age of 21 years, while the intensity of use among users remained unchanged. These estimates are robust to a variety of specifications and also remain robust across different subsamples. Copyright © 2014 John Wiley & Sons, Ltd.