This study confirms previously reported alterations in the levels of circulating β-endorphin in chronic pain. It further brings up the importance of genetic and sex factors in sensitivity to opioids. Thus, in this study we observed that the minor allele of the OPRM1 gene tended to be associated with increased opioid sensitivity in chronic pain, that there were sex differences in β-endorphin levels and an increased incidence of opioid side effects in patients with the minor allele of the ABCB1 transporter gene, and that there was a relationship between high opioid sensitivity and the presence of the major allele of the CACNA2D2 gene.
