Absorption and bioavailability of rectally administered morphine in women

21 healthy women undergoing gynaecological operations received rectal premedication with morphine 0.3 mg/kg body weight. Plasma concentrations of morphine were followed for 4 h by a GC/MS technique. In most patients the peak plasma concentration was reached after 30 min; the mean peak plasma level of morphine was 18 ng/ml (range 8.5-57 ng/ml). The bioavailability of rectal morphine was determined in 6 patients, who received an i.m. injection of morphine at a second operation. The mean bioavailability of rectal morphine was 31% (range 12%-61%). None of the patients showed any clinical sign of respiratory depression, and there was no increase in end-tidal carbon dioxide tension measured in 5 patients operated under spinal block.