Methotrexate is a prescription medication used to treat certain types of cancers. It is also used to treat psoriasis, polyarticular juvenile idiopathic arthritis (PJIA), and rheumatoid arthritis (RA).Methotrexate belongs to a group of drugs called antimetabolites.ral/Injectable: Methotrexate is a prescription medication used to treat the following conditions: 1  severe psoriasis. 2  rheumatoid arthritis. 3  polyarticular juvenile idiopathic arthritis (PJIA). 4  breast cancer. 5  certain types of head and neck cancer. 6  lung cancer. 7  advanced non-Hodgkin lymphoma (NHL).