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. 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. Methotrexate, formerly known as amethopterin, is a chemotherapy agent and immune-system suppressant. It is used to treat cancer, autoimmune diseases, and ectopic pregnancies. It is known as a disease-modifying anti-rheumatic drug (DMARD), because it not only decreases the pain and swelling of arthritis, but it also can prevent damage to joints and long-term disability. Methotrexate comes either as pills or as a subcutaneous injection. 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. 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 is one of the most effective and widely used medications for treating inflammatory types of arthritis. It’s also one of the safest arthritis drugs, despite a common misconception among many patients that methotrexate is highly toxic. 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. Easy-to-read patient tips for methotrexate covering how it works, benefits, risks, and best practices. 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.