The most common long-term problems after rectal surgery are bowel problems. These problems can include needing to pass bowel movements a lot, loose or mushy stools, inability to fully clear your bowels, and/or poor control of gas and stool. The investigators believe that a major cause of these problems is too much bacteria in the bowel and treatment with antibiotic tablets will hopefully help improve these bowel problems. In order to test this idea, the Colorectal Surgery Service of Memorial Sloan Kettering Cancer Center is sponsoring a clinical trial. This trial will compare the antibiotic rifaximin and a placebo (a harmless tablet that has no effect) in the treatment of these bowel problems. Following this we will attempt to see if another antibiotic metronidazole also helps to treat these bowel problems. Both Metronidazole and rifaximin are well established drugs that have minimal side effects.