Pharmacokinetics and analgesic effects of i.m. and oral ketamine

The pharmacokinetics and analgesic effects of i.m. and oral ketamine in a dose of 0.5 mg kg-1 were determined in six healthy volunteers. Analgesia was measured by the submaximal effort tourniquet test. Following both routes of administration, ketamine prolonged the period of pain-free ischaemic exercise. Pain thresholds were increased at 15 min and 30 min after i.m. injection and at 30 min after oral ketamine. The plasma ketamine concentration associated with analgesia was 150 ng ml-1 following the i.m. dose, but only 40 ng ml-1 after the oral dose. Oral administration was, however, associated with much greater concentrations of the metabolite norketamine, which may have contributed to the analgesic effect.