Morphine suppresses the oxytocin response in breast-feeding women

The activity of opiate-mediated regulatory mechanisms of oxytocin secretion during breast-feeding was studied by the administration of either morphine, naloxone or placebo to women prior to the commencement of breast-feeding. Seventeen healthy women in the first week after delivery who had established lactation were randomized to receive either intravenous morphine 5 mg (n = 6), naloxone 2.4 mg (n = 6) or a placebo, sterile water (n = 5), which was given prior to commencement of breast-feeding. Oxytocin levels were measured by radioimmunoassay prior to initiation of breast-feeding and then at 2-min intervals until the feed was complete. Breast-feeding produced a significant rise in oxytocin levels in the control and naloxone groups but no significant rise in the patients given morphine. There was a significant reduction in oxytocin response following morphine administration when compared to placebo but not between naloxone and placebo. In conclusion, oxytocin secretion to breast-feeding is inhibited by exogenous morphine when compared to a control group but the administration of naloxone did not produce a significant difference from control.