Colitis can be on the right or left side, and generally the left lower abdomen is the preferred site for colitis. The colon is divided into four parts: the ascending colon, the transverse colon, the descending colon and the sigmoid colon, where the ascending colon is on the right side of the abdomen, the descending and sigmoid colon on the left side, and the transverse colon in the upper abdomen. Therefore, when a patient develops colitis, both the right and left sides are possible. Generally speaking, colitis involving the left half of the colon is more common, so the pain site is mainly on the left side, mostly manifested as pain in the left lower abdomen or a sensation of anal swelling, occasionally mucous blood stools, and the pain will be relieved after defecation. If the scope of the lesion increases, the pain on the right side may also occur, but it is relatively rare.