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The aim of this prospective observational study was to evaluate the influence of OPRM1 polymorphism on the analgesic efficacy (including visual analog scale [VAS] scores and requirement for rescue analgesia) of a standard dose of intrathecal morphine.An Italian cohort of 63 parturients, scheduled for elective cesarean section at a tertiary University Hospital, received spinal anesthesia with hyperbaric bupivacaine and morphine 100 mcg. For the first 48 hours in the postoperative period the patients received acetaminophen 1 g IV q6hr. Incident pain was treated with ketorolac 30 mg IV. Every 6 hours the following parameters were registered: VAS at rest, VAS during movements, postoperative nausea and vomiting, pruritus, and rescue analgesic medications requirements. Age and anthropometric data, number of pregnancies, educational level, OPRM1 genotype, were also obtained.Of the 63 patients enrolled, 45 (71%) were homozygous genotype A/A (118A group), whereas 18 carried the G variants of OPRM1 (A/G or G/G) (118G group). No significant differences in analgesic rescue doses’ administration and in incidence of moderate/severe postoperative pain (VAS>3) between the 2 groups were observed. Pruritus was more frequent in the 118A group than in the 118G group in the first 24 hours of the postoperative period.In the Italian population participating in this study there was a different incidence of pruritus in the postcesarean period in response to intrathecal opioids related to OPRM1 gene polymorphism, but not of postoperative pain.