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Background Pain due to oral mucositis (OM) is a major problem during concurrent chemoradiotherapy (CCRT) in nasopharyngeal carcinoma (NPC) patients. Methods We enrolled 56 NPC patients receiving CCRT and allocated them into two groups: moderate pain group ( n = 27) and a severe pain group ( n = 29) according to the degree of pain reported (moderate = numerical rating scale (NRS) score 4–6 or severe = NRS score 7–10) at initiation of controlled-release oxycodone (CRO) treatment. Results Total dose of CRO was significantly higher in severe pain patients than in moderate pain patients (791.60 ± 332.449 mg vs. 587.27 ± 194.940 mg; P = 0.015). Moderate pain patients had significantly better quality of life ( P = 0.037), lower weight loss ( P = 0.030) and more active CCRT response (90.9% vs. 64.0%; P = 0.041). Although 24-h pain control rate was comparable in the two groups (85.2% vs. 86.2%; P = 0.508), the moderate pain group score eventually stabilized at ~ 2 vs. 3 in the severe pain group ( P < 0.001); the titration time to reach bearable pain (NRS ≤ 3) was also significantly shorter in moderate pain patients (2.45 ± 0.60 days vs. 3.60 ± 1.98 days; P = 0.012). Incidence of adverse events was comparable in both groups. Conclusions The study findings suggest that early introduction of low-dose CRO at the moderate pain stage could help reduce the total dose required, provide better pain control, improve quality of life, and enhance CCRT response.