To investigate the comparability of the treatment groups, the baseline characteristics of the subjects randomized to the methadone alone and the co-prescribed heroin treatment conditions were compared on key variables in the areas of demographic background (gender, age, ethnicity), components of the primary outcome measure (MAP-HSS, SCL-90, days illegal activities, days contact non-drug-using person), and lifetime and current substance use variables (years of use, and days of use in past month for heroin, methadone, cocaine, amphetamines, benzodiazepines, alcohol, and poly drug use). No significant differences (Bonferroni correction; overall p-value <0.10) between the treatment groups were found on any of these key variables.