We did not find any significant interaction of naltrexone with the OPRM1 genotypes of interest in this study. Surprisingly, there was also no significant main effect of naltrexone despite such an effect being evident in similar previous studies (O’Malley et al., 2002; Drobes et al., 2003; Anton et al., 2004) and no main effect of OPRM1 genotype on alcohol consumption despite reports to the contrary (Ray and Hutchinson, 2004; Ray and Hutchinson, 2007). The reasons for this are unclear, but might include slight differences in study population, the a priori genetic analysis and selection influencing expectancies, or just a type II error (an a priori power calculation predicted 80% chance of finding a main effect of naltrexone and a 80% chance of detecting a naltrexone by OPRM1 interaction with this sample size). Given this lack of “main effect” of naltrexone, the lack of an interaction between genotype and medication might be considered less meaningful. On the other hand, it is also possible that the OPRM1 gene by naltrexone interaction might not be as salient in this younger, less severe, and
