Over the past 25 years in the United States, enforcement of drug prohibition has
expanded dramatically. Over the same period, however, the trends in drug
production and consumption have been essentially flat, and the real, purityadjusted prices of both cocaine and heroin have more than halved. This
combination of facts raises questions about the effectiveness of prohibition
enforcement, and it constitutes a puzzle that is interesting to explain. In this
paper the authors document these facts and explore possible explanations. They
do not claim to provide a complete answer, but shed light on which explanations
are likely to be important.