Objectives: This study evaluates the relationship between the various words expressing pain and the efficacy of opioid analgesics, in order to provide information on the appropriate selection of analgesic drugs for cancer pain relief. A new and simplified evaluation list for the assessment of pain, including the common words used to express pain, was developed accordingly. Methods: The words expressing pain were classified into three categories based on the Guidelines for Drug Therapy in Cancer Pain. These words were grouped into clusters according to the efficacy of opioid drugs, as expressed by the patients. On the basis of these results, an evaluation list was developed, and its validity was assessed by clinical pharmacists and cancer patients experiencing pain. Results: On the basis of the classification of the words, their relationship with opioid efficacy was summarized, and the results of the cluster analysis were found to be helpful for the development of a new pain evaluation list. Conclusion: Patients use various words to express their pain. By classifying and clustering these words according to the guidelines and the efficacy of opioid drugs, we proposed a practical pain evaluation list for appropriate selection of analgesic drugs in patients with cancer.