Methadone has been studied most intensively in heroin addiction pharmacogenetics research.[12] Several variants in ABCB1 CYP2B6 OPRD1 genes were indentified association with methadone plasma levels in the candidate gene study.[12,13] In addition, 2 GWAS identified CYP2B6 and OPRM1 genes significant association with methadone dose.[14,15] Because of chemical structure of methadone, the CYP2C19 isozyme metabolizes methadone R-enantiomer[16] and CYP2B6 isozyme metabolizes methadone S-enantiomer[17,18] in the liver. Here, OPRD1 is linked to heroin dependence.