methotrexate, drug used in halting the growth of actively proliferating tissues. Introduced in the 1950s, it is used in the treatment of leukemia, psoriasis, and non-Hodgkin's lymphoma. By binding to an enzyme that controls the metabolism of folic acid, methotrexate interferes with synthesis of nucleic acids and therefore with tissue cell reproduction. Because of its immunosuppressive qualities, it is also used in the treatment of rheumatoid arthritis.

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