Methotrexate and formerly known as amethopterin, is an antimetabolite and antifolate drug used in treatment of cancer and autoimmune diseases. It acts by inhibiting the metabolism of folic acid. Methotrexate replaced the more powerful and toxic antifolate aminopterin, and the two should not be confused with each other.

Methotrexate was originally used, as part of combination chemotherapy regimens, to treat many kinds of cancers. It is still the mainstay for the treatment of many neoplastic disorders including acute lymphoblastic leukemia.

More recently it has come into use as a treatment for some autoimmune diseases, including ankylosing spondylitis, Crohn's disease, psoriasis, psoriatic arthritis, and rheumatoid arthritis (see disease-modifying antirheumatic drugs). A parallel use with TNFα blockers as infliximab or etanercept has been shown to markedly improve symptoms.

Although not indicated for this use, methotrexate is also sometimes used (generally in combination with misoprostol) to terminate early pregnancies, particularly ectopic pregnancies. In the case of early missed miscarriage (particularly a blighted ovum), in which fetal demise has occurred but the body has not expelled the fetus, methotrexate may be used to help the body begin the miscarriage process. This is not a termination, because in this case, the fetus is no longer living.

It is also sometimes used to treat a rare condition called Behçet's disease where it is taken weekly, along with folic acid daily.