Sep 24, 2018 Methotrexate is an immune suppressive drug used to treat certain types of cancer as well as a number of autoimmune disorders, including severe psoriasis and rheumatoid arthritis (RA). Ironically, a number of studies have suggested that methotrexate may, in fact, increase the risk of cancer in people with RA, including melanoma, leukemia, myeloma, lung cancer, and lymphoma.