Racemic ketamine has an oral bioavailability of approximately 17%. When administrated orally in a dose of 0.5mg/kg the plasma ketamine concentration of ketamine was 40 ngml-1 (for the same i.m. dose the plasma ketamine concentration was 150 ng ml-1 ). Oral administration is associated with much greater concentrations of the metabolite norketamine, which may have contributed to the analgesic effect. Orally administered ketamine undergoes extensive first pass metabolism, primarily via N-de-methylation, resulting in low ketamine concentrations and high norketamine concentrations in blood and tissue. The plasma levels at which analgesia is achieved are 0.15 μg/ml following intramuscular administration and 0.04 μg/ml after oral administration. This difference may be explained by a higher norketamine concentration due to first-pass metabolism. This main metabolite apparently contributes to the antinociceptive effect Oral ketamine is more potent (30 to 40%) that subcutaneous route because first pass metabolism converts ketamine to an active analgesic metabolite.