Abstract Body (Do not enter title and authors here): Background: Obesity is an adverse prognostic factor for breast cancer (BC). While liraglutide and semaglutide, derived from antidiabetic glucagon-like peptide-1 receptor agonists (GLP1RA), are established weight loss drugs known for their cardiovascular safety in the general obese population, their safety in BC patients undergoing cardiotoxic antineoplastic therapies remains less understood. Objective: To investigate the mortality and cardiovascular outcomes related to GLP1RA used for weight loss among the highly susceptible BC females. Methods: This retrospective cohort study was based on the TrinetX network, comprising real-time health records from over 60 US healthcare organizations. The study population included obese females (≥18 years old) who received at least three prescriptions of either exclusive GLP1RA (liraglutide or semaglutide) or non-GLP1RA weight loss drugs (phentermine, bupropion-naltrexone, orlistat) upon BC diagnosis between 01/01/2015 and 01/01/2022. The index date was the initiation of systemic anti-cancer treatment, including chemotherapy (anthracycline, taxane, or cyclophosphamide), and/or anti-HER2 agents (trastuzumab, pertuzumab, or lapatinib). We used 1:1 propensity score-matching (PSM) to balance demographics, BMI, comorbidities, concurrent medications for cardiovascular diseases and metabolic disorders, BC receptor status and prior treatment, and levels of serum cholesterol (LDL, HDL) and hemoglobin A1c. Cox regression models were applied to compute hazard ratios (HR) and 95% confidence intervals (CIs). Follow-up continued until 05/24/2024, or until adverse cardiovascular events or death occurred. Results: Among 380 PSM-balanced pairs, the GLP1RA group (mean age: 57.6±11) showed significantly lower risks of all-cause mortality (HR 0.593, 95% CI 0.457-0.77), acute heart failure (HR 0.496, 95% CI 0.286-0.859), angina pectoris (HR 0.566, 95% CI 0.343-0.936), and acute stroke (HR 0.661, 95% CI 0.46-0.951) than the non-GLP1RA group (mean age: 57.5±12). Otherwise, no significant difference was shown in acute myocardial infarction (HR 0.787, 95% CI 0.495-1.251), pulmonary hypertension (HR 0.777, 95% CI 0.55-1.099), or arrhythmias (HR 0.829, 95% CI 0.566-1.215). Conclusion: Compared to non-GLP1RA weight loss agents, GLP1RA offers better survival and cardiovascular protection for BC patients undergoing cardiotoxic therapies. Future studies are warranted to validate the safety of these weight loss drugs during the BC treatment continuum.