Pethidine is commonly used in single doses as a preoperative medication or in multiple doses as an analgesic. The clinical consequences of altered disposition are more likely to result from its analgesic use. Correlations between plasma pethidine concentration, analgesia and side effects such as respiratory depression, have been established, but considerable overlap exists between concentrations producing therapeutic and non-therapeutic effects. The current practice of intermittent pethidine administration (intravenous, intramuscular and oral) for analgesia results in fluctuations in pethidine plasma concentrations which are associated with incomplete pain relief and side effects. Continuous intravenous infusion of pethidine may avoid these difficulties. Changes in pethidine disposition have been observed in patients with liver disease and in the elderly. Measurement of plasma pethidine concentrations may be helpful as an aid to the management of such patients. In renal disease, metabolites may accumulate and cause side effects.