Chloramphenicol is a unique antimicrobial agent; however, because of its tendency to cause blood dyscrasias in humans, 2 related drugs have been developed. Thiamphenicol is less effective but safer than chloramphenicol; florfenicol, a thiamphenicol derivative, is significantly more active in vitro than chloramphenicol against many pathogenic strains of bacteria. Florfenicol is approved for use in cattle.