Comparative effects of clarithromycin and erythromycin on the normal intestinal microflora.

10 healthy volunteers received 250 mg of clarithromycin orally q 12 h for 7 days and 10 other volunteers 1000 mg of erythromycin ethylsuccinate orally q 12 h for 7 days. Stool specimens were collected before, during and after antibiotic administration. In the clarithromycin group, the numbers of streptococci and enterobacteria decreased among aerobic microorganisms while in the erythromycin group streptococci, enterococci and enterobacteria decreased and staphylococci increased during antibiotic administration. The anaerobic intestinal microflora was also affected. The alterations were more pronounced in the volunteers receiving erythromycin than in those having clarithromycin.