Methotrexate (4-amino-10-methylfolic acid) is a folic acid antagonist that inhibits the reduction of folic acid and the growth of tissue cells. Methotrexate competitively inhibits the enzyme dihydrofolate reductase, preventing the reduction of dihydrofolate to tetrahydrofolate, which is necessary for DNA synthesis and cell replication. As a result of polyglutamation of methotrexate caused by the folylpolyglutamylate enzyme, the duration of the cytotoxic effect of the active substance in cells increases.