Background: Intravesical gemcitabine (iGEM) use among patients with high-risk non-muscle invasive bladder cancer (HR NMIBC) has shown effectiveness towards preventing or delaying tumor recurrence, although Bacillus Calmette-Guérin (BCG) remains the standard of care for newly diagnosed HR NMIBC. This retrospective cohort study investigated iGEM users and reported their treatment patterns among SEER-Medicare enrollees newly diagnosed with HR NMIBC patients naïve (BCG-N) or exposed (BCG-Exp) to BCG therapy. Methods: Patients enrolled inMedicare Part A & B fee-for-service enrolled, with HR NMIBC, defined by Tis, Ta, T1, N0, M0 and ≥65 years with iGEM use between 2008-2020 were evaluated. Patients with missing TNM stage, low/ intermediate risk NMIBC, or other primary cancers were excluded. Index date was defined as the first claim of iGEM after HR NMIBC diagnosis, regardless of their prior BCG use. Patients were followed until the earliest event of death, end of Medicare advantage enrollment, or data availability end (12/31/2020). Demographic and clinical characteristics were reported in the baseline period (12-month pre-index) and treatment patterns were reported calendar year post-index date for BCG-N and BCG-Exp cohorts. Results: The analysis included 277 BCG-N and 402 BCG-Exp patients receiving iGEM. In both cohorts, mean age was >75 years, ≥80% were male, and >90% were White. The mean (SD) national cancer index comorbidity index score was 0.85 (0.7) for BCG-N and 0.63 (0.7) for BCG-Exp. Intravesical therapy use beyond a perioperative instillation during the baseline period was observed in 27% of BCG-N and 14% of BCG-Exp patients; docetaxel (17% BCG-N, <1% BCG-Exp) and mitomycin (12% BCG-N, 14% BCG-Exp) were most common. The median number of iGEM doses in the BCG-N group was 6 (IQR 1,8) with mean (SD) of 28 (43) days of retreatment interval. In BCG-Exp, the median number of iGEM doses was 6 (IQR 2,9) with mean (SD) of 23 (26) days of retreatment interval. The table shows increasing iGEM use in both BCG-N and BCG-Exp groups, with 70% of BCG-N in years 2019 through 2020, and 22-30% of BCG-Exp patients receiving iGEM in years 2017 through 2020. Conclusions: Recent BCG shortages may have played a significant role in recent increased uptake of iGEM in both BCG-N and BCG-Exp cohort among HR NMIBC patients. Further information on the efficacy and safety outcomes of iGEM vs BCG in various HR NMIBC populations are needed.