With poor health and widespread drug problems in the U.S. prison population, post-prison drug use provides an important measure of both public health and social integration following incarceration. The authors study the correlates of drug use with data from the Boston Reentry Study (BRS), a survey of men and women interviewed four times over the year after prison release. The BRS data allow an analysis of legal and illegal drug use, and the correlation between them. The authors find that illegal drug use is associated with histories of drug problems and childhood trauma. Use of medications is associated with poor physical health and a history of mental illness. Legal and illegal drug use are not strongly correlated. Results suggest that in a Medicaid expansion state where health coverage is widely provided to people leaving prison, formerly-incarcerated men and women use medications, not illegal drugs, to address their health needs.