Methotrexate is a folic acid derivative belonging to the class of cytotoxic agents known as antimetabolites. It acts during the S phase of the cell cycle and competitively inhibits the enzyme dihydrofolate reductase, thereby preventing the reduction of dihydrofolate to tetrahydrofolate, which is essential 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 malignant tissues proliferate more rapidly than normal, methotrexate can disrupt their development without causing irreversible damage to healthy tissues.