Clearance of continuous intravenous morphine for severe cancer pain

Four cancer patients with intractable pain received continuous morphine infusions in doses of 15-275 mg/h for a time period ranging from 4 to 27 days. Serum morphine concentrations were determined periodically following adjustments in infusion rates. As doses were changed and continued at static hourly rates, serum morphine concentrations were relatively constant 20 hours and beyond the time of the respective change, thus suggesting morphine elimination half-lives of less than or equal to 4 hours. High doses did not influence the time required to achieve steady-state concentrations. Steady serum morphine concentrations corresponded with hourly morphine doses in a parallel manner. High interpatient variabilities in clearances and steady-state serum morphine concentrations were noted. These data suggest that at morphine infusions up to 275 mg/h elimination pathways permit handling of increasing concentrations of morphine without nonlinear blood level increases. Also, marked interpatient and intrapatient variations in patient dose requirements were noted.