In this review, four candidate gene studies assessing methylation patterns at CpG islands associated with the OPRM1 promoter region were identified. Two studies by the same group characterized associations between specific CpGs in Caucasians [118] and subsequently in Hispanics and African-Americans maintained on methadone [119], finding associations differed between ethnicities. Overall, differences were fairly small, with the largest reported as 7.1% at CpG −25 in Hispanic former heroin addicts versus controls, as compared to the largest being 5.6% at CpG +12 in African-Americans, who had significantly higher baseline rates of methylation across the CpG island as compared to Caucasians and Hispanics. Of note, the former study used both lymphocytes and whole blood, whereas in the latter lymphocytes only were used.
