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Importance: In addition to protective effects on the cardiovascular system, statins may reduce the risk of breast cancer recurrence owing to potential anti-inflammatory benefits. Given that patients with breast cancer in Asia are relatively younger at diagnosis and most are free from traditional cardiovascular risk factors, it is uncertain whether the use of statins can improve survival., Objective: To investigate the association of statin use with cancer- and noncancer-associated survival in patients with breast cancer., Design, Setting and Participants: This cohort study used the Taiwanese National Health Insurance Research Database and National Cancer Registry to identify patients diagnosed with breast cancer from January 2012 to December 2017. Age, cancer stage, anticancer therapies, comorbidities, socioeconomic status, and cardiovascular drugs were matched by propensity score method. Statistical analyses, including Cox proportional hazards models, were performed from June 2022 to February 2023. The mean (SD) follow-up duration was 4.10 (2.96) years., Interventions: Patients receiving statins within 6 months before the diagnosis of breast cancer were compared with those not receiving statins., Main Outcomes and Measures: Outcomes included death, heart failure, and arterial and venous events., Results: Overall, 7451 patients (mean [SD] age, 64.3 [9.4] years) treated with statins were matched with 7451 nonusers (mean [SD] age, 65.8 [10.8] years). Compared with nonusers, statin users had a significantly lower risk of all-cause death (adjusted hazard ratio [HR], 0.83; 95% CI, 0.77-0.91; P < .001). Notably, the risk reduction was mainly attributed to cancer-related death (adjusted HR, 0.83; 95% CI, 0.75-0.92; P < .001). Only a small number of patients died of cardiovascular causes, and the ratios were similar between statin users and nonusers. No significant differences were observed in cardiovascular outcomes, including heart failure and arterial and venous events, between statin users and nonusers. Using a time-dependent analysis, statin users also presented a significantly lower risk of cancer-related death (adjusted HR, 0.28; 95% CI, 0.24-0.32; P < .001) than nonusers, and notably, the risk was even lower in high-dose statin (HDS) users compared with non-HDS users (HDS users: adjusted HR, 0.84; 95% CI, 0.73-0.98; P = .002; non-HDS users: adjusted HR, 0.79; 95% CI, 0.68-0.91; P = 001)., Conclusions and Relevance: In this cohort study of Asian patients with breast cancer, statin use was associated with a reduced risk of cancer-associated death rather than cardiovascular death. Our findings provide evidence to support the use of statins in patients with breast cancer; however, randomized studies are necessary.