Genomic DNA was collected and isolated from buccal swabs using standard procedures (Freeman et al., 1997; Walker et al., 1999). The A118G SNP in the OPRM1 gene was assayed using a commercially available instrument and TaqMan assays (Applied Biosystems, Foster City, California). To ensure accurate data, genotypes for all participants were determined by two independent laboratory technicians blinded to participants’ characteristics. In addition, a randomly selected subset of samples (10%) was assayed again to assess reliability (kappa = 1.0).
