Compare the Efficacy and Safety of Intranasal Esketamine in Chronic Opioid Refractory Pain

Inclusion Criteria: * Patient with refractory cancer pain, this pain defined when: * Multiple evidence- based biomedical therapies used in a clinically appropriate and acceptable fashion have failed to reach treatment goals that mainly include adequate pain reduction and/or improvement in daily living functioning activities. * Patients' functional activities do not allow a quality of life which is acceptable and/or pharmaceutical therapies have resulted in intolerable adverse effects. * Psychiatric disorders and psychosocial factors that could influence pain outcomes have been assessed and appropriately addressed * Cancer pain classified as chronic (persistent or recurrent pain lasting longer than 3 months), and currently refractory despite optimized analgesic therapy including an opioid. • Optimized analgesic therapy is arbitrarily defined as: oral morphine equivalent of 60 mg/d or more (or another strong opioid at optimized dose) plus at least one adjuvant analgesic drug, for at least 2 weeks. * No increase in baseline long acting opioid dose or addition of a new adjuvant analgesic drug within 2 weeks prior to study entry * Ability to communicate the intensity of pain using the NPRS pain scale ranging from (0 as no pain to 10 with severe pain). * Ability to give fully informed written consent. * Expect survival more than 3 months. Exclusion Criteria: * History of allergy or intolerance to esketamine or ketamine. * History of allergy to disinfecting products conta…