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The influence of two different doses of oral naltrexone on the adverse effects and the analgesia of epidural morphine were compared in a double-blind, placebo-controlled study. Forty-five patients undergoing cesarean section were provided postoperative analgesia with 4 mg epidural morphine. Five minutes later they received 6 mg naltrexone, 9 mg naltrexone, or placebo as an oral solution. Pain relief was assessed by the Visual Analog Scale (VAS) and by direct questioning of the patients. Requirement for additional analgesics and side effects were noted. Respiratory effects of epidural morphine and naltrexone were assessed using the ventilatory responses to CO2 and by monitoring O2 saturation (Spo2) using pulse oximetry. All patients in the placebo group had adequate analgesia. One of the 15 patients who received naltrexone 6 mg had inadequate analgesia versus five of the 15 patients who received naltrexone 9 mg (P < 0.05), 9 mg versus placebo. Ten patients (67%) in the placebo group had pruritus while no patient in the 6 mg naltrexone group and one patient in the 9 mg group experienced mild pruritus (P < 0.05), placebo versus other two groups. The CO2 response slopes were depressed compared to control values from 6–16 h in the placebo group, from 6–12 h in the 6 mg naltrexone group. No significant depression was noted in the 9 mg naltrexone group. The authors conclude that oral naltrexone 6 mg significantly reduces the incidence of pruritus associated with epidural morphine without affecting analgesia and that 9 mg naltrexone is associated with shorter duration of analgesia than 6 mg naltrexone.