Clarithromycin use in patients who are receiving theophylline may be associated with increased serum theophylline concentrations. Clarithromycin may increase carbamazepine concentrations. Active metabolite of terfenadine was 3-fold higher when co-administered with clarithromycin. Concomitant administration of erythromycin and digoxin has been reported to result in elevated digoxin level.