Ketamine in cancer pain: an update.

Ketamine has been shown to have potent analgesic properties at low dosages. Bioavailability is high when it is given parenterally, but low after oral or rectal administration. Active metabolites should account for part of the analgesic effect of ketamine during long-term oral administration. NMDA receptor inhibition by ketamine may reflect a wind-down phenomenon, and should alleviate NMDA-related neuropathic pain, reversing the rightward shift of the opioid-response curve. A synergistic effect between ketamine and opioids has been observed in cancer pain patients who have lost an analgesic response to high doses of morphine. Further studies need to be carried out to confirm the benefits of ketamine in cancer pain, and to determine the best route of administration, dosages and the incidence of side effects.