Methotrexate is an antimetabolite and antifolate agent with antineoplastic and immunosuppressant activities. Methotrexate binds to and inhibits the enzyme dihydrofolate reductase, resulting in inhibition of purine nucleotide and thymidylate synthesis and, subsequently, inhibition of DNA and RNA syntheses. Methotrexate also exhibits potent immunosuppressant activity although the mechanism(s) of actions is unclear.

Methotrexate is approved by the Food and Drug Administration (FDA) to be used by itself to treat the following types of gestational trophoblastic tumors, Chorioadenoma destruens, Choriocarcinoma, Hydatidiform mole. Methotrexate is also approved to be used alone or with other drugs to treat acute lymphoblastic leukemia (ALL) that has spread to the central nervous system (CNS) or to keep it from spreading there, and to treat the following, breast cancer, certain types of head and neck cancer, lung cancer, advanced non-Hodgkin lymphoma (NHL), advanced mycosis fungoides (a type of cutaneous T-cell lymphoma), osteosarcoma that has not spread to other parts of the body, following surgery to remove the primary tumor. Methotrexate is also approved to treat the following non-cancer conditions, rheumatoid arthritis, severe psoriasis.