From the National Cancer Institute-
"The use of antibiotics should be considered as an important element  
affecting microbiota composition. It has been demonstrated, and our  
present study has confirmed, that after antibiotic treatment the  
bacterial composition in the gut never returns to its initial  
composition,” “Thus, our findings raise the possibility that the  
frequent use of antibiotics during a patient’s lifetime or to treat  
infections related to cancer and its side-effects may affect the  
success of anti-cancer therapy.”