Ketamine and its role in cancer pain

Ketamine has two enantiomers; S-ketamine appears to have fewer psychotomimetic side-effects and better analgesic activity in neuropathic pain. Although long used to provide analgesic cover for painful procedures in children, such as marrow aspiration, and in postoperative pain via epidural infusion with morphine, its specific action on NMDA receptors explains its synergism with morphine in neuropathic pain. Doses of 48-700 mg per day by continuous subcutaneous infusion, or 0.1-0.2 mg/kg body weight intravenously have provided good analgesia in patients with severe pain despite high doses of morphine. Ketamine injectable solution given orally in doses up to 0.5 mg/kg per 12 hours also provides good pain relief in neuropathic pain, probably partly via its active metabolite norketamine. Low dose midazolam or haloperidol decreases the incidence of distressing dissociative side-effects, Ketamine's safety in haemodynamically unstable patients and the preservation of airways reflexes makes it particularly useful in the terminally ill frail cancer patient, although it can cause tachycardia and excessive salivation. Other uses include analgesia for incident pain when a patient is moved or transported and for control of headache from cerebral metastases. Ketamine may affect the immune function and tumour response to some chemotherapy.