Initial assessment for the inclusion criteria was conducted through a telephone interview, after which eligible participants were invited to the laboratory for an in-person assessment session. Upon arrival at the lab, participants read and signed an informed consent form, provided information about quantity and frequency of drinking episodes over the past year, and provided a saliva sample for DNA analyses. Participants were prospectively genotyped prior to being invited to the physical exam and alcohol challenge sessions in order to oversample for individuals who carry at least one copy of the G allele of the A118G SNP of the OPRM1 gene, thus resulting in exclusion of 64 A allele homozygotes. Genotypes for kappa and delta opioid receptor genes were not considered prospectively. Prior to participating in the ethanol infusion session, participants received a physical examination at the General Clinical Research Center (GCRC) in order to ensure that participants were in good physical health and were medically eligible to take the study medication and to participate in the alcohol challenge. Of the total of 124 participants (39 women) who were screened in
