Purpose: Trastuzumab, a potent anti-human epidermal growth factor receptor 2 (HER2) monoclonal antibody, is conditionally reimbursed by the Taiwan National Health Insurance (NHI) for HER2-positive breast cancer (BC). Trastuzumab-induced cardiotoxicity studies have well characterized heart failure (HF) but fewer addressed arrhythmia, particularly the association of potential life threatening atrial fibrillation (Af) is poorly characterized. We aimed to study the trastuzumab-related risk of Af and HF using the claimed data of Taiwan NHI. Methods: A nationwide retrospective cohort of patients with BC from the Taiwan NHI reimbursement database from January 2007 to December 2016 was analyzed. Propensity score matching and competing risk model analysis were used for adjusting confounding concurrent medication or comorbidities and competing events. The HF study was used to validate the method used. Results: For Af, 12,472 trastuzumab users were matched with 12,472 non-trastuzumab users. For HF, 12,241 trastuzumab users and 12,241 non-users were enrolled. We found that trastuzumab users had significantly worse HF-free survival but not Af-free survival than non-trastuzumab users. In the competing risk analysis, the use of trastuzumab did not increase the risk of Af (hazard ratio [HR] 0.76, P = 0.0006) but was associated with HF (HR 1.19, P = 0.0052). The risk trends among stratifications by comorbidities and concurrent medication remained in similar directions for both Af and HF. Conclusion: Trastuzumab in real-world practice was associated with an increased risk of HF, but was not associated with an increased risk of Af in BC patients. Trastuzumab-induced arrhythmogenic effects may be masked by concurrent heart-protecting measures, more prominent roles of comorbidities or concurrent medications under real-world settings. Further studies are required.