Relief of pain by infusion of morphine after operation: does tolerance develop?

To see whether continuous intravenous infusion of opiates provides more effective postoperative relief of pain than conventional intramuscular injection these regimens were compared in a prospective double blind trial. Thirty patients undergoing elective cholecystectomy were allocated randomly to receive an infusion of morphine or an infusion of placebo (control group) for 24 hours. Both groups were allowed supplementary morphine boluses as requested. During the first 48 hours after operation the degree of pain was almost identical between the groups. Surprisingly, the group that was given the infusion of morphine received as much supplementary morphine as the control group during the first 24 hours and appreciably more during the 24 hours after the infusion had been withdrawn. Nausea and vomiting were more prevalent among the patients given the infusion of morphine. These results suggest that continuous infusion of morphine may be an inferior regimen to intermittent bolus administration in the relief of postoperative pain. This may be explained by the development of tolerance in patients who received the infusion of morphine.