Used primarily in chemotherapy to treat various types of cancer, including leukemia, breast cancer, and lung cancer. It works by inhibiting cell growth by blocking the action of dihydrofolate reductase, an enzyme involved in DNA synthesis. Also used in the treatment of autoimmune diseases such as rheumatoid arthritis and psoriasis, where it helps reduce inflammation and slow disease progression. Commonly administered orally or by injection, it is effective in suppressing overactive immune responses and controlling abnormal cell proliferation.