Intestinal side effects after antibiotic therapy are frequent. Mostly, harmless diarrhea disappears after cessation of therapy without inducing colitis; however, changing of the intestinal flora sometimes leads to colonization of the colon by toxin-producing strains of Clostridium difficile, inducing sometimes severe pseudomembranous colitis. A rapid correct diagnosis by anamnesis, clinical signs, endoscopical aspect and demonstration of toxin in the stool allows an efficient treatment. The therapeutic modalities, especially in recurrency, are delineated. In addition, the unusual and etiologically still unknown illness of penicillin-induced segmental hemorrhagic colitis will be discussed as well.