Methotrexate (formerly Amethopterin) is an antimetabolite originally used to treat many kinds of cancer. More recently it has come into use as a treatment for some autoimmune diseases, including psoriasis and arthritis. It can be taken orally or by injection. Possible side effects can include anemia, increased risk of bruising, and nausea.

Chemically methotrexate is N-[4-(2,4-di[[amino-6-pteridinyl)methyl]methylamino]benzoyl]-L-glutamic acid.