Chronic anal fissure is one of the most common causes of anal pain and surgical therapy is the treatment of choice. There is scarce information regarding the prophylactic effects of oral metronidazole on postoperative complications of anal fissure. The objective of this study was to determine the effects of metronidazole as a prophylactic measure for postoperative anal fissure complications. In a numerical randomized clinical trial, 311 patients with anal fissure were randomized into two groups. The group which received prophylactic oral metronidazole was compared to the control group regarding wound dehiscence, bleeding, discharge, and pain after one and two weeks of operation. One hundred fifty-six patients with mean+/-SD age of 36+/-7.3 years were in metronidazole group and 155 patients with mean+/-SD age of 38+/-7.1 years were in the control group. Regarding the studied outcome variables, the patients in both groups did not have any significant differences after one and two weeks. Our results did not support the prophylactic use of metronidazole in reducing postsurgical complications after internal sphincterotomy of anal fissure.