Therapies that effectively suppress graft-versus-host disease (GVHD) without compromising graft-versus-leukemia/lymphoma (GVL) effects is important in allogeneic hematopoietic stem cell transplantation (allo-HSCT) for hematopoietic malignancies. Geranylgeranylacetone (GGA) is a main component of teprenone, a gastric mucosal protectant commonly used in clinical practice. In preclinical models, GGA suppresses proinflammatory cytokines, including interleukin (IL)-1β, IL-6, and tumor necrosis factor-α (TNF-α), which are associated with GVHD as well as induces thioredoxin-1 (Trx-1), which suppresses GVHD while maintaining GVL effects. Here, we investigated whether the addition of teprenone to standard GVHD prophylaxis could reduce the cumulative incidence of severe acute GVHD (aGVHD) without attenuating GVL effects. This open-label, randomized clinical trial enrolled 40 patients (21 control and 19 teprenone group) who received allo-HSCT between May 2022 and February 2023 in our institution. Patients in the teprenone group received 50 mg of teprenone orally thrice daily for 21 days from the initiation of the conditioning regimen. The cumulative incidence of severe aGVHD by day 100 after allo-HSCT was not significantly different in the two groups (27.9 vs. 16.1%, p = 0.25). The exploratory studies revealed no obvious changes in Trx-1 levels, but the alternations from baseline in IL-1β and TNF-α levels at day 28 after allo-HSCT tended to be lower in the teprenone group. In conclusion, we could not demonstrate that teprenone significantly prevented the development of severe aGVHD. Discrepancy with preclinical model suggests that appropriate dose of teprenone may be necessary to induce the expression of antioxidant enzymes that suppress severe aGVHD. Clinical Trial Registration number:jRCTs 061210072.