Complete guide to methotrexate: what it treats, proper dosage, common side effects, drug interactions, and important safety information you need to know. 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, formerly known as amethopterin, is a chemotherapy agent and immune-system suppressant. It is used to treat cancer, autoimmune diseases, and ectopic pregnancies. 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. 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. 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. 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 (MTX) use can be associated with a variety of adverse effects over a wide range of severity (table 1); the risk of most adverse effects is influenced by the MTX dose and treatment regimen. 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 interferes with the growth of certain cells of the body, especially cells that reproduce quickly, such as cancer cells, bone marrow cells, and skin cells. Methotrexate is used to treat leukemia and certain types of cancer of the breast, skin, head and neck, lung, or uterus.