Used in combination chemotherapy, particularly in the anticancer drug S-1, where it enhances the efficacy of 5-fluorouracil (5-FU) by inhibiting its degradation. Gimeracil acts as a potent inhibitor of dihydropyrimidine dehydrogenase (DPD), the enzyme responsible for breaking down 5-FU in the body. By blocking this enzyme, it prolongs the half-life and increases the concentration of 5-FU, allowing for sustained antitumor activity at lower doses. This results in improved therapeutic outcomes and reduced side effects in the treatment of cancers such as gastric, colorectal, and head and neck cancers. Its role is purely supportive, optimizing the pharmacokinetics of the active cytotoxic agent without direct antineoplastic action.