Methotrexate injection is used alone or in combination with other medications: to treat certain types of acute lymphocytic leukemia (ALL) in adults and children; to treat or prevent meningeal leukemia (cancer in the covering of the spinal cord and brain) in adults and children; to treat certain types of non-Hodgkin's lymphoma (types of cancer that begin in a type of white blood cells that normally fights infection) in adults and children; to treat osteosarcoma (cancer that forms in bones) after surgery to remove the tumor in adults and children; to treat cutaneous T-cell lymphoma (CTCL, a group of cancers of the immune system that first appear as skin rashes); to treat breast cancer in adults; to treat certain cancers of the head and neck in adults; to treat gestational trophoblastic tumors (a type of tumor that forms inside a woman's uterus when pregnant) in adults; to treat rheumatoid arthritis (RA; a condition in which the body attacks its own joints, causing pain, swelling, and loss of function) in adults; to treat polyarticular juvenile idiopathic arthritis (pJIA; a type of childhood arthritis that affects five or more joints during the first six months of the condition, causing pain, swelling, and loss of function) in children; and to treat severe psoriasis (a skin disease in which red, scaly patches form on some areas of the body) in adults. Methotrexate is in a class of medications called antimetabolites. Methotrexate treats cancer by slowing the growth of cancer cells. Methotrexate treats psoriasis by slowing the growth of skin cells to stop scales from forming. Methotrexate may treat rheumatoid arthritis and polyarticular juvenile idiopathic arthritis by decreasing the activity of the immune system.