Male patients with the major allele of OPRM1 had higher concentrations of β-endorphin (26.8 ± 3.25 fmol/ml) than male patients with the minor allele (24.2 ±2.96 fmol/ml; p < 0.05). There were no differences in that respect among female patients or in the control groups (Table 2). Furthermore, a borderline significant association was found in an age and sex adjusted model of OPRM1 and opioid sensitivity (r = 0.188, adjusted R2 = 0.035 p = 0.106).
