Methotrexate, formerly known as amethopterin, is a chemotherapy agent and immune-system suppressant. It is used to treat cancer, autoimmune diseases, and ectopic pregnancies. Find out how methotrexate treats rheumatoid arthritis, psoriasis, Crohn's disease and other autoimmune conditions, and how to take it. NHS medicines information on methotrexate – what it's used for, side effects, dosage and who can take it. 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. Learn about the side effects of methotrexate, from common to rare, for consumers and healthcare professionals. 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. 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. 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. 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 is used to treat certain types of cancer and rheumatoid arthritis. Learn about side effects, drug interactions, dosages, warnings, and more.