There is increasing interest in the pharmacogenetic basis for explaining differences between patients in treatment outcome among methadone-treated subjects. Most studies have focused on genetic polymorphisms related to methadone pharmacokinetics and, to a lesser extent, those genes implicated in the pharmacodynamics of methadone.This study aimed to investigate the associations between response to methadone maintenance treatment (MMT) and polymorphisms in genes coding for the OPRM1 opioid receptor, the metabotropic glutamate receptors GRM6 and GRM8, the nuclear receptor NR4A2, the photolyase enzyme cryptochrome 1 (CRY1), and the transcription factor myocardin (MYOCD), which have previously been associated with the risk of opioid dependence disorder.The study used an association, case-control design, conducted in the setting of an MMT program in a drug abuse outpatient center in Barcelona, Spain. We recruited 169 opioid-dependent patients (diagnosed according to the Diagnostic and Statistical Manual of Mental Disorders [4th Edition] criteria) receiving MMT. The inclusion criteria included Caucasian ethnicity, being enrolled in MMT for at least 6 months, and receiving a stable methadone dose for the previous 2 months. The exclusion criteria included language-related barriers, severe cognitive impairment, or any medical disorder that would interfere with the research assessments. Single nucleotide polymorphism (SNP) variants in several candidate genes and regions were genotyped: MYOCD (rs1714984), GRM8 (rs1034576), CRY1 (rs1861591), GRM6 (rs953741), OPRM1 (rs1074287), NR4A2 (rs1405735), and the intergenic variants rs965972 (1q31.2) and rs1867898 (2q21.2). MMT response status was assessed by the number of opioid-positive controls detected by random urinalysis in the previous 2 months. We used the chi-squared test and p-value for the allele frequencies of the eight SNPs in responders versus nonresponders, and multivariate logistic regression analyses to examine associations between genotypes in the responder and ronresponder groups under codominant, dominant, and recessive models of inheritance.A final sample of 116 opioid-dependent patients were included and classified as methadone responders (n = 83) and nonresponders (n = 33), according to illicit opioid use detection in random urinalysis. The responders and nonresponders showed similar demographic and clinical characteristics. All SNPs were in Hardy-Weinberg equilibrium. Subjects carrying the AA genotype at rs1861591 (CRY1; Chr 12: 105941056 G>A) had a higher risk of being nonresponders (odds ratio [OR] = 2.99; 95% CI 1.14, 7.85; p = 0.035), although this difference disappeared with multiple testing corrections. Patients carrying the A allele at rs1714984 (MYOCD; Chr 17: 12558425 G>A) had an increased risk of being nonresponders only if they were also carriers of the AG genotype at rs953741 (GRM6; Chr5: 178262451 A>G) [OR = 10.83; 95% CI 2.52, 46.66; p = 0.006].A positive association was observed between response to methadone and two variants in the genes MYOCD and GRM6. A pharmacogenetic epistatic effect between SNPs in MYOCD and GRM6 appears to modulate inter-individual variations in MMT response.