May cause hypoprothrombinemia and bleeding (use with particular caution in patients >40 years of age). May cause significant bone marrow depression (agranulocytosis). Aplastic anemia, thrombocytopenia, and leukopenia may also occur. Use with caution in patients receiving other drugs known to cause myelosuppression, particularly agranulocytosis. Thyroid hyperplasia or carcinoma may occur with prolonged usage. Associated with rare but severe dermatologic reactions. Discontinue in presence of unexplained fever. Rare, severe hepatic reactions (hepatic necrosis, hepatitis) may occur. Associated with a variety of autoimmune reactions (including lupus-like syndrome), glomerulonephritis and interstitial nephritis with acute renal failure, rare reports of interstitial pneumonitis, and rarely with development of ANCA-positive vasculitis or leukocytoclastic vasculitis.