Background: Administration of some medications, such as antibiotics, antacids, and probiotic supplementation, may affects the treatment outcomes of immune checkpoint inhibitors (ICIs), through the change of gut microbiota in cancer patients. Even when ICIs are combined with cytotoxic chemotherapy (Chemo), a use of such medications is expected to be a potential biomarker. However, the impacts of medications that affect gut microbiota on the efficacy of Chemo alone is unclear. Methods: We retrospectively analyzed patients who received fluoropyrimidines and platinum-based therapy for advanced gastric cancer in our institutions between January 2010 and December 2019. Progression-free survival (PFS) and overall survival (OS) by history of antibiotic/antacid/probiotic supplementation, were analysed. Results: A total of 101 pts were analyzed. Median age was 66 years (range, 30-81), female/male; 36/65, 1st-line regimen SP/XP/SOX/XELOX/FOLFOX; 52/2/39/1/7. In all patients, The median PFS for first-line treatment was 5.8 months (95% CI: 5.0-7.3) and the median OS was 15.5 months (95% CI: 12.5-18.6).Univariate analysis for PFS of first-line treatment did not show a significant difference in antibiotic use (median 7.6 vs 5.5 months, HR: 0.79 [95%CI: 0.50-1.24], p=0.31), oral antacids (median 6.6 vs 5.8 months, HR: 0.96 [95%CI: 0.61-1.54], p=0.89) and probiotic supplementation (median 9.1 vs 5.5 months, HR: 0.7 [95%CI: 0.38-1.26], p=0.24). Similarly in OS, the use of antibiotics (median 18.4 versus 13.4 months, HR: 0.85 [95%CI: 0.53-1.37], p=0.49), oral antacids (median 16.3 vs 13.8 months, HR: 1.15 [95%CI: 0.70-1.88], p=0.58), and oral probiotic supplementation (median 20.2 vs 13.8 months, HR: 0.77 [95%CI: 0.42-1.40], p=0.40), with no statistically significant differences. Conclusions: In this study, there were no significant differences in PFS and OS between patients administered with or without antibiotics, antacids, or probiotic supplementation. Future studies are expected to investigate the impacts of these medications on the treatment outcomes of ICIs+Chemo combination.