Enhanced nephrotoxicity w/ cisplatin. Increased risk of severe neurological adverse events w/ cytarabine (IV). Antagonized effect w/ L-asparaginase. Increased plasma levels of mercaptopurine. Elevated & prolonged serum levels w/ NSAIDs & salicylates. Increased adverse effects w/ gold, penicillamine, hydroxychloroquine, sulfasalazine. Elevated plasma levels w/ proton pump inhibitors. Diminished renal tubular transport by ciprofloxacin & probenecid. Reduced renal clearance w/ penicillins & sulfonamides. Decreased intestinal absorption w/ oral antibiotics eg, tetracycline, chloramphenicol & nonabsorbable broad spectrum antibiotics. Increased bone marrow suppression w/ trimethoprim/sulfamethoxazole. Increased toxic effects w/ antiprotozoal pyrimethamine. Increased risk of hepatotoxicity w/ other hepatotoxic agents eg, leflunomide, azathioprine, sulfasalazine, retinoids. Potentiated effect on folate metabolism w/ nitrous oxide anesth. Decreased response w/ folic acid-containing vit. Induced ulcerated skin lesions w/ amiodarone. Increased toxicity w/ sulfonylureas, aminobenzoic acid, salicylates, phenylbutazone, phenytoin, sulfonamides & some antibiotics eg, penicillins, tetracycline, pristinamycin, probenecid, chloramphenicol. Increased risk of pancytopenia w/ leflunomide. Skin cancer w/ psoralen + PUVA therapy. Decreased clearance of theophylline. Bone marrow suppression & decreased folate levels w/ triamterene.