Objective:To explore the effects of dexmedetomidine (DEX) on postoperative pain, oxidative stress and adverse reactions in patients undergoing radical mastectomy.Methods:A total of 90 patients with breast cancer who received radical surgical treatment in our hospital from Jun. 2022 to Jun. 2023 were prospectively included as research objects and randomly divided into 3 groups with 30 patients in each group. DEX group was applied before, during and after surgery, respectively. The levels of pain visual analogue scale (VAS), Richmonation sedation score (RASS), superoxide dismu-tase (SOD) and malondialdehyde (MDA) were recorded.Results:The recovery time and extubation time in preoperative and intraoperative DEX group were significantly lower than those in postoperative DEX group, and the awakening time and extubation time in preoperative DEX group were significantly lower than those in intraoperative DEX group ( F value was 48.62 and 53.98, respectively, P<0.001). At 1 h, 6 h and 12 h after surgery, the VAS and RASS scores of patients in the preoperative and intraoperative DEX group were significantly lower than those in the postoperative DEX group, compared with those in the intraoperative DEX group. The VAS and RASS scores in the DEX group were significantly decreased ( F value: 62.34, 55.24, 69.26, 36.82, 24.20, 39.97, P<0.001). At 24h after surgery, there was no significant difference in VAS and RASS scores among the three groups ( F value was 0.45 and 0.81, respectively, P value was 0.613 and 0.418). Immediately after surgery, 24 h after surgery, 72 h after surgery, the SOD level of DEX group was significantly higher than that of DEX group before and during surgery ( F value was 29.37, 33.24, 10.35, P<0.001). MDA levels were significantly lower than those in postoperative DEX group ( F value was 30.52, 41.27, 8.26, P<0.001). There was no significant difference in the incidence of postoperative adverse reactions among all groups ( P>0.05) . Conclusion:Preoperative and intraoperative application of DEX can reduce postoperative pain and oxidative stress in breast cancer patients, help patients recover quickly after surgery, and preoperative application is superior to intraoperative application.