Methotrexate is a type of drug called an antimetabolite. It treats rheumatoid arthritis by decreasing the activity of your immune system, treats psoriasis by slowing the growth of skin cells and treats cancer by slowing the growth of cancer cells. Methotrexate treats certain leukemias and lymphomas, mycosis fungoides, rheumatoid arthritis, polyarticular juvenile idiopathic arthritis in children, and severe psoriasis in adults. Learn about side effects, interactions, and indications. More than 900,000 people in the U.S. take methotrexate, often as a first-line treatment for rheumatoid arthritis (RA). It’s also used at much higher doses to treat some forms of cancer. Overview: Methotrexate is used to treat certain types of arthritis, cancer, and severe psoriasis. Common side effects include nausea or vomiting, upset stomach, and mouth sores. Serious side. Learn about the side effects of methotrexate, from common to rare, for consumers and healthcare professionals. Methotrexate, formerly known as amethopterin, is a chemotherapy agent and immune-system suppressant. It is used to treat cancer, autoimmune diseases, and ectopic pregnancies. Methotrexate tablet is used alone or together with other medicines to treat several types of cancer, such as breast, head and neck, lung, blood (eg, acute lymphoblastic leukemia or ALL), bone, lymph node (eg, mycosis fungoides, relapsed or refractory non-Hodgkin lymphoma), or uterus cancer. Based on published reports of female infertility after methotrexate, advise females of reproductive potential that methotrexate can cause impairment of fertility and menstrual dysfunction. Facts for patients and caregivers about the SMARD drug Methotrexate (Rheumatrex, Trexall, Otrexup, Rasuvo), such as usages, common dosages, safety tips, and possible side effects. Methotrexate injection is used alone or together with other medicines to treat several types of cancer, such as breast, head and neck, lung, blood, bone, lymph node, and uterus cancers.