Women were well represented in this sample (17.3% and 15.6% of ethnic Bulgarian and Roma cases, respectively; close to the proportion observed in populations of heroin users). The slightly higher frequency of the 118G allele in heroin dependent Roma women raises the possibility of sex-specific differences, as suggested, for example, by studies of the effect of this polymorphism on reduced sensitivity to pain (Fillingim et al., 2005) and alcohol-dependence (Kim, 2009). Moreover, in a mouse model, Mague et al. found sex-specific effects of the 118A>G polymorphism on drug mediated behaviors, such as reductions in the rewarding properties of morphine and the aversive components of naloxone-precipitated morphine withdrawal (Mague et al., 2009).
