<qa>  What is the name of the receptor antagonist that improves analgesia in patients withLong-term opioid therapy is commonly administered for the management of severe cancer pain. Increasing doses of opioids are titrated against effects until analgesia is achieved or intolerable adverse effects occur. Ketamine, an N-methyl-D-aspartate NMDA receptor antagonist, has been reported to improve analgesia in patients with uncontrolled pain receiving high doses of opioids. This study aims at determining the effectiveness of ketamine as an adjuvant to opioids in relieving cancer pain. 