Methotrexate is a folic acid antagonist and belongs to the class of cytotoxic agents of the antimetabolite class. It acts during the S phase of the cell cycle and competitively inhibits the enzyme dihydrofolate reductase, thus preventing the reduction of dihydrofolate to tetrahydrofolate, which is necessary for DNA synthesis and cell replication. Actively proliferating tissues, such as malignant tumors, bone marrow, fetal cells, oral and intestinal mucosa, and bladder cells, are usually more sensitive to methotrexate. Since the rate of proliferation of malignant tissues is faster than normal, methotrexate can disrupt their development without causing irreversible damage to healthy tissues.