Patients w/ CHF, chronic obstructive lung disease or concomitant infections, & those taking erythromycin, troleandomycin, lincomycin & other antibiotics of the same group, allopurinol, cimetidine, propranolol & anti-flu vaccine. Phenytoin, other anticonvulsants & smoking. Hypoxemia, hyperthyroidism, liver & renal disease, history of peptic ulcer. Pregnancy. Elderly.