Clinical pharmacokinetics applied to patients with intractable pain: studies with pethidine.

The minimum effective analgetic blood concentration (MEAC) of pethidine following intravenous administration was identified in 3 patients with intractable pain. In two patients this value remained the same whether the pethidine was given intravenously, orally or rectally. In the third patient, whose eneral bioavailability was only 20%, the MEAC was not obtained. However, intramuscular administration reliably achieved the MEAC and was useful clinically. During the study period of 3-12 months, the individual patient's MEAC remained similar. Two patients developed dependence on, and tolerance to, pethidine but neither the dependence nor the tolerance appeared to be related to the MEAC. These studies confirm the importance of clinical pharmacokinetic measurements in the investigation and treatment of patients with intractable pain.