Methotrexate 50 mg injection is used to treat recipients with leukemia and certain types of cancer of the lung, breast, skin, head and neck. The methotrexate 50 mg injection is also used for the treatment of severe Psoriasis and Rheumatoid Arthritis in adult patients, and used for treating active polyarticular-course juvenile Rheumatoid Arthritis in children. This medication is often recommended when other agents have not been effective.