Methotrexate, a prescription medicine, is in a class of drugs called antimetabolites. It used in the treatment of certain cancers, severe psoriasis, and adult rheumatoid arthritis. In the treatment of certain types of cancers and psoriasis, Methotrexate interferes with DNA synthesis, repair, and cellular replication. The mechanism of action in rheumatoid arthritis is unknown; it may affect immune function.