Data from surveys of arrestees and the household population in the U.S. suggest there is only modest overlap among demand for the big three expensive illegal drugs (cocaine/crack, heroin, and methamphetamine). In particular, the number of chronic users of these substances (defined as consuming on four or more days in the previous month) is only about 10% below a naïve estimate obtained by simply summing the numbers of chronic users for each of the three substances, while ignoring polydrug use entirely. This finding does not gainsay that polydrug use is common or important. One would estimate greater overlap if one adopted a more expansive definition of polydrug use (e.g., has the individual ever used another substance at any time in their life) or a more expansive list of substances (e.g., allowing marijuana or alcohol to count as one of the substances makes polydrug use seem much more common). However, it does suggest that when focusing on the illegal drug markets that generate the most crime, violence, and overdose death in the U.S., one can usefully think of three more or less separate markets populated at any given time by largely distinct populations of drug users.