In this prospective, observational study we explored whether A118G single nucleotide polymorphism in the human
mu-opioid receptor (MOR) gene could explain the inter-individual differences in
opioid analgesic requirements in patients with
acute postoperative pain and
chronic pain. The frequency of the wild-type A118 MOR (major) and variant G118 MOR (minor) alleles in the subjects with chronic, noncancer
pain (n = 121) and
opioid-naïve subjects with
acute postoperative pain (n = 101), serving as the control group, were examined. The relationships among the A118G MOR genotype,
opioid requirements, and the numerical
pain score were analyzed in both groups. The frequency of the minor allele was significantly lower in the subjects with
chronic pain when compared with the group with
acute postoperative pain (0.079 versus 0.158; P = 0.009 by chi2 test). No statistically significant association was observed between the presence of A118G MOR polymorphism and the average
postoperative pain score or the doses of
morphine used in the immediate postoperative period. In the high-quartile,
opioid utilization,
chronic pain patients, the homozygotic carriers of the major allele required significantly higher
opioid dose than did the carriers of the minor allele. The results indicate that although the presence of the minor allele does not appear to affect
opioid analgesic use in
acute postoperative pain, the minor allele is less common in
chronic pain patients, especially in those requiring higher doses of
opioid analgesics.