Plasma morphine concentration after intrathecal administration of low doses of morphine

Plasma morphine concentration was measured in eight patients who received morphine 0.02 mg kg-1 intrathecally. The concentration was less than 0.5 ng ml-1 at 2 min and 10 min, 2 +/- 1 ng ml-1 at 30 min, 3 +/- 2 ng ml-1 at 1 h, 6 +/- 4 ng ml-1 at 3 h, 5 +/- 3 ng ml-1 at 12 h, 0.8 +/- 0.2 ng ml-1 at 24 h and less than 0.5 ng ml-1 at 36 h. The small plasma morphine concentrations we observed after intrathecal administration of morphine indicated that analgesia is a result of an action of morphine on opiate receptors, either spinal or cerebral.