Opioids are very useful medications to reduce suffering of cancer patients such as refractory pain and dyspnea. We physicians have to use opioids to have good management of pain and suffering associated with cancer including management of side effects caused by opioids. Opioids couple opioid receptors and affect several pharmacological effects. Other than analgesic effect, opioids have some side effects of constipation, nausea and vomiting, respiratory depression. In this chapter, I take important side effects of constipation, nausea and vomiting and respiratory depression. Next, serotonin syndrome caused by tramadol combined with anti-depressants is remarked as assignable syndrome. As advancing in chemotherapy for cancer treatment, cancer survivors live longer with opioid therapy. We have to pay attention to the side effects and another dysfunction caused by long use of opioids. It is important that we physician use opioids effectively to keep activity of daily living(ADL) of patients and families as team approach.