Methadone, the long-term major treatment of opioid addiction, is a mu opioid receptor agonist and a weak N-methyl-D-aspartic acid (NMDA) receptor antagonist. Part of the large inter-individual variability in drug response may be accounted for by genetic factors. Successful methadone treatment for opiate dependence relies in part on dosage optimization. Several pharmacogenetics studies of methadone have been performed to date, but only a few have studied variants in the opioid system genes that are relevant to this review. No association was found between OPRM1 118A>G or OPRD1 921T>C and response to MMT and methadone dose in a study of 238 European patients (Crettol et al. 2008).
