Randomization Prior to group assignment, participants were stratified to an experimental group in a 1:1 ratio according to whether they attended the workplace on >85% of days during the 4-week induction period, provided >1 opiate positive urine sample during the final 2 weeks of the induction period, and provided >75% cocaine-positive urine samples during the 4-week induction period. Eligible participants were assigned to one of two experimental groups (i.e., Prescription or Contingency) using an urn randomization method that ensured all levels of each stratification variable were evenly distributed across the groups. This research design originally proposed three experimental groups; therefore, four participants were randomized into a third experimental group that required participants to ingest naltrexone under staff observation and provide an opiate and cocaine-negative urine sample to maintain maximum pay in the workplace. Due to logistical difficulties encountered early in the study, it became clear that it would not be possible to randomize a sample size large enough to support a 3-group comparison within the planned study duration. A second power analysis was completed to determine the appropriate sample size for a 2-group comparison (described below) and the third experimental group was eliminated. Data from participants randomized into the third group have not been included in these analyses.