Background: Recent studies have suggested a role for the tumor microbiome in conferring chemoresistance to gemcitabine (GEM) in patients with pancreatic ductal adenocarcinoma (PDAC). One proposed resistance mechanism is the presence of bacterial enzymes that metabolize GEM, but not 5-fluorouracil (5-FU). We therefore assessed the impact of antibiotic receipt in the month before and after initiation of either GEM- or 5-FU-based chemotherapy on PDAC overall survival (OS). Methods: We used the SEER-Medicare database to identify adults >66 years old with stage IV PDAC treated with either first-line GEM or 5-FU between 2007-2017. Antibiotic receipt was defined as a claim for >5 days of oral antibiotics or >1 intravenous antibiotic in the month before or after chemotherapy initiation. Demographic and clinical characteristics were compared using chi-squared and student t-tests, and univariable survival was assessed using Kaplan-Meier methods. Propensity scores (PS) were created to account for potential allocation bias in the receipt of antibiotics. We fit PS-adjusted Cox proportional-hazard models to evaluate the association between antibiotic receipt and OS. Results: We identified 3,990 stage IV PDAC patients treated with first-line GEM-based (n = 3,219) or 5-FU-based (n = 771) chemotherapy, of whom 2,318 (58.1%) received antibiotics. In univariable analyses, antibiotic receipt in the month surrounding chemotherapy initiation was associated with improved median OS in patients treated with GEM (214 days vs 199 days, p = 0.04) but not in patients treated with 5-FU (255 days vs 284 days, P = 0.14). In PS-adjusted multivariable analyses, the significant association between antibiotic receipt and improved OS in patients who received GEM remained (HR 0.89, 95% CI 0.83-0.96). However, no significant association between antibiotic receipt and OS was seen in patients treated with 5-FU (1.10, 95% CI 0.94-1.28). Conclusions: Antibiotic receipt in the month before or after chemotherapy initiation was associated with improved OS in stage IV PDAC patients who received GEM but not 5-FU. These findings support the concept that antibiotics may impact the activity of GEM-metabolizing bacteria proposed to confer chemoresistance. Further studies are needed to characterize the potential role for antibiotics in PDAC treatment.