Rate of formation of norpethidine from pethidine

The concentrations of norpethidine in blood were determined in patients recovering from abdominal surgery following both multiple i.m. injections (i.m. group) and the i.v. infusion (i.v. group) of pethidine. The rate of administration of pethidine averaged 25 mg h-1 in both groups. Concentrations of norpethidine were generally small compared with those of pethidine although the ratio of norpethidine to pethidine concentrations increased steadily over the 2 days studied. The rate of increase of blood concentration of norpethidine was 0.013 micrograms ml-1 h-1 in both groups of patients. No significant toxic effects were observed.