Relationship between plasma concentration of morphine and analgesic effectiveness.

Morphine is a key drug for cancer pain management. In this study, we analyzed plasma morphine concentration in cancer patients who received continuous morphine drip and/or oral administrations of morphine: (1) The plasma concentration of morphine varied widely in patients whose pain was satisfactorily controlled at a constant dosage of morphine. The absolute value of the plasma concentration of morphine necessary for effective pain control could not be estimated; (2) When MS Contin was given at the doses 20 mg, 30 mg, 40 mg, the plasma concentration of morphine increased with increasing dose. Thus, when the dose was increased according to the severity of pain, the plasma concentration of morphine increased and in turn an analgesic effect was obtained; (3) In those patients who had difficulty in taking oral preparations and/or blocked intestines, plasma concentration of morphine following oral administration was relatively low causing an unsatisfactory analgesic effect. However, by changing from oral administration to continuous drip infusion, the plasma concentration of morphine became higher and pain relief was obtained; (4) Continuous drip infusion of morphine progressively increased plasma concentration of morphine in parallel with the increase in the dose of morphine if the patients had no pleural effusion, ascites, and/or oedema. In contrast, plasma morphine concentration in patients with pleural effusion, ascites, and/or oedema was about half of that observed in patients who have normal distribution area. The rapid development of pleural effusion and ascites lowers the blood level of morphine; (5) To use the plasma concentration of morphine as an index for the analgesic effect, it is essential to develop a method to measure the plasma concentration of morphine rapidly.