Increased excretion & decreased therapeutic effectiveness of lithium. May enhance sensitivity & toxic potential of cardiac glycosides. May exhibit synergistic toxicity w/ ephedrine & other sympathomimetics. Enhanced effects of oral anticoagulants. Increased serum conc w/ cimetidine, high-dose allopurinol, OCs, propranolol, ciprofloxacin, erythromycin & troleandomycin. Decreased serum conc w/ rifampin & phenytoin. Decreased clearance w/ methotrexate. Increase cardiotoxic effects w/ α-adrenergic antagonist eg, isoproterenol.