Clarithromycin acts by inhibiting microsomal protein synthesis in susceptible organisms mainly by binding to the donor site on the 50S sub- unit of the bacterial ribosome and preventing translocation to that site.
Clarithromycin is active against most Gram-positive bacteria and Chlamydia, some Gram-negative bacteria and Mycoplasmas.
Clarithromycin?s activity is the same as, or greater than, that of erythromycin in vitro against most Gram-positive bacteria. Clarithromycin is more acid stable than erythromycin and therefore, is better tolerated. Clarithromycin has twice the activity of erythromycin against H. influenzae. Most species of Gram-negative bacteria are resistant to clarithromycin because of failure to penetrate to the target.