consistently. Other pharmacogenetic studies have examined treatment response for alcohol or opiate dependence. Variability in genes that are involved in methadone metabolism, such as CYP3A4 and CYP2D6, as well as genetic variation in P-glycoprotein (ABCB1, MDR1), for which methadone is a substrate, and DRD2 may alter the methadone dosage required for maintenance in heroin addicts (51–53). However, methadone is a complex drug for which the pharmacokinetic and pharmacodynamic responses are not well understood, and the genetic factors that underlie the variability in its response are not well known.
