Effect of erythromycin stearate on serum theophylline concentration in patients with chronic obstructive lung disease.

The effect of erythromycin stearate (ES) on serum theophylline concentration (STC) was studied in 15 male adult subjects taking maintenance oral theophylline for clinically stable chronic obstructive lung disease. Steady-state trough STCs were measured before and after two- and seven-day courses of ES, 500 mg every six hours. For the group as a whole, there was no significant increase in trough STC after either course of ES therapy, but five of 15 subjects exhibited increases in trough STC ranging from 4 to 8 micrograms/ml (mean increase of 5.4 micrograms/ml) within 48 to 96 hours of beginning ES therapy. Clinical theophylline toxicity developed in one of these subjects. Based on this study and others in the literature, erythromycin preparations may induce a clinically significant and potentially toxic increase in STC in a variable proportion of patients taking maintenance theophylline therapy.