Severe pain from mucositis resulting from concomitant chemoradiotherapy (CCRT) is commonly treated with opioid analgesics. However, the period of use of opioid analgesics differs among individuals. We performed a retrospective cohort study of 46 patients who were treated with long-term opioid analgesics after CCRT for head and neck cancer. Among these patients, opioid analgesics were used for a median of 30 days. A comparative study was conducted between patients with long-term use of strong opioid analgesic drugs and those with short-term use. A significantly prolonged use of opioid analgesic agent was associated with cetuximab use (TPF vs. S-1 vs. Cmab, 35.0 vs. 44.1 vs. 180.7, p≤0.001). There were no psychiatric symptoms such as dependence and delirium. When patient background and chemotherapy selection are known, the possibility of severe oropharyngeal mucositis and the potential need for long-term opioid analgesics can be evaluated. Consideration of reduction or discontinuation according to pain after the end of treatment is important.