Methotrexate

Methotrexate (Trexall, Rheumatrex) is a folate antagonist that inhibits DNA synthesis and cell replication, giving it both immunosuppressive/anti-inflammatory effects at low doses and cytotoxic effects at high doses used in oncology. At low doses, it is a cornerstone disease-modifying antirheumatic drug (DMARD) for rheumatoid arthritis and is also used for psoriasis and psoriatic arthritis. Critically, low-dose methotrexate for these conditions is dosed once WEEKLY, not daily — daily dosing errors have caused serious, sometimes fatal, toxicity. It carries boxed warnings for hepatotoxicity, bone marrow suppression, and severe skin reactions, and requires regular lab monitoring; folic acid is often co-prescribed to reduce side effects.