Patients w/ HTN, heart disease, hypoxaemia, hyperthyroidism, chronic right ventricular failure, CHF, liver & renal disease, history of peptic ulcer; COPD or concomitant infections. Concomitant use w/ erythromycin, troleandomycin, lincomycin & other antibiotics of the same group, allopurinol, cimetidine, propranolol & anti-flu vaccine. Pregnancy. Elderly.