On a weight basis, rifampin is the antibiotic most active against Chlamydia trachomatis. Because of concerns about the ease with which resistance to rifampin emerges in vitro, few clinical trials of rifampin for the treatment of human chlamydial infections have been done. In those few studies, however, rifampin has been found to be as effective as tetracyclines for the treatment of C. trachomatis infection of the male urethra or for the topical treatment of trachoma.