Background: Gut microbiome dysbiosis is correlated with graft-versus-host disease (GVHD) in allogeneic stem cell transplant (allo-SCT) patients. In the allo-SCT population, antibiotics have been associated with increased risk for GVHD mortality and relapse due to loss of gut obligate anaerobes. It has been shown that antibiotics may negatively impact the efficacy of checkpoint inhibitors for patients with solid tumors. Based on these studies, we performed a retrospective analysis to determine if antibiotic treatment affects outcomes of multiple myeloma (MM) patients after autologous SCT (ASCT). Methods: This is a single institution retrospective study at Hackensack University Medical Center. A list of consecutive MM patients treated from 1/2012 to 12/2015 was obtained and an electronic medical record review of the first 217 who received ASCT was performed. Baseline characteristics, treatment history, transplant course and antibiotic treatment (including β-lactams, fluoroquinolones, macrolides, metronidazole, and vancomycin) were reviewed. Prophylactic antibiotics were excluded. Response was defined using the IMWG criteria. Median progression free survival (PFS) and overall survival (OS) were estimated using the Kaplan-Meier method. Log rank tests were used to compare the difference in survival between stratified groups. The LASSO cox regression analysis method was used for multivariate analyses of PFS and OS. Results: Of the 217 patients, 205 patients were available for analysis. Median age at ASCT was 61. β-lactams were associated with decreased median PFS (1.95 vs 4.77 years (yrs), p < 0.01) and decreased median OS (7.51 vs 13.45 yrs, p = 0.01). Multivariate analysis adjusting for lasso-selected age, gender, complete remission (CR) after ASCT, and ISS demonstrated independent progression risk associated with β-lactam use (HR = 2.00, 95% CI, 1.28–3.12, p < 0.01). β-lactams were associated with worse OS in multivariate analysis adjusting for lasso-selected age, gender, CR after ASCT and high risk cytogenetics (HR = 1.89, 95% CI, 1.07–3.40, p = 0.03). Conclusions: In this preliminary study, β-lactams predicted for decreased PFS and OS compared to patients who did not receive β-lactams in MM patients undergoing ASCT. The study was limited by its retrospective nature but demonstrates one of the first evaluations of antibiotic effect on the ASCT population in MM. Prospective studies evaluating the impact of antimicrobials on patient outcomes and the gut microbiome are ongoing.