 Pain is a most common symptom and it has a high impact on quality of life in cancer patients. Many cancer patients have received opioid therapy, but also many of them have suffered from side effects of opioids. Drowsiness and confusion are common side effects of opioids. Caffeine is a well known psychostimulant,and it is widely used as an analgesics. Thus, the investigators aimed to prove the efficacy of intravenous caffeine administration as an adjuvant analgesics to opioids. At the same time, the investigators tried to find that the side effects of opioids could be ameliorated by caffeine. 