Objective: This study was performed to identify single nucleotide polymorphisms 1236C>T (dbSNP rs1128503), 2677G>T / A (dbSNP rs2032582), and 3435C>T (dbSNP rs1045642) of the ABCB1 gene in patients with substance abuse opiates for methadone replacement therapy and to assess the relationship with baseline dose.