Analgesic responses to morphine and placebo in individuals with postoperative pain

Abstract

The effects of placebo and varying doses of i.v. morphine were studied in 74 patients. All patients underwent extraction of impacted mandibular 3rd molars. Two hours after onset of anesthesia all patients received a placebo (i.v. saline). One hour after the placebo administration each patient received either a 2nd placebo or 4, 6, 8 or 12 mg of morphine, double blind, via a hidden i.v. line. Pain level was evaluated 50 min after morphine administration using a visual analog scale. Pooled data from all patients produced a dose-response curve asymptotic by 8 mg. The mean pain relief following the 2nd placebo was between that obtained following hidden administration of 4 and 6 mg of morphine. When pain level reports for individuals were plotted 2 unexpected features appeared. No patient reported complete relief even at the highest dose of morphine (12 mg). Pain level reports 50 min following each dose of morphine tended to be in 2 clusters. Within each cluster the average pain was independent of the dose of morphine administered. In groups receiving progressively higher doses of morphine, the percentage of patients within the low pain level cluster increased. These latter observations are most consistent with the concept that there is a step component for narcotic analgesia.