Ulcerative colitis (UC) is increasingly common, its exact etiology has not been elucidated, and it is a lifelong disease. Mesalazine (5-aminosalicylic acid, also known as 5-ASA) is the main drug used to treat UC. Most of the patients with UC in China are mild and do not need surgery and can have a good quality of life with the control of mesalazine. However, some patients are not seriously ill, but they are always poorly treated, why is this? UC usually starts in the rectum and spreads upward, invading the sigmoid colon, descending colon, splenic flexure, transverse colon, hepatic flexure, ascending colon, cecum, and also “inverted ileitis”. The extent of UC determines the treatment plan and whether oral or topical treatment is indicated. Oral mesalazine is difficult to reach the most distal parts of the GI tract: the sigmoid colon and rectum. These two areas are precisely where UC is most often involved. Therefore, suppositories are available for proctitis, and enemas are used for left-sided colitis; these local treatment modalities are often preferred. Although the lesion is in the rectum, combined oral therapy is more effective. For extensive colitis (beyond the splenic flexure), there is no doubt that “oral therapy + local therapy” is required. In general, topical treatment is essential. Mesalazine has a therapeutic effect through direct contact with the intestinal mucosa, so it has a range of action and a dose requirement. The range of action varies from dosage form to dosage form. Different drug dosage forms are released at different sites and exert their local therapeutic effect. Mild and moderate UC may be treated with mesalazine, while severe UC will require hospitalization and further therapeutic measures. The treatment of UC may seem simple, but there is actually a lot going on inside. The treatment plan differs depending on the disease. For example, in the acute phase of treatment, how to maintain remission, whether to use hormones or biological agents, the dose of hormones, when to stop, how to judge the efficacy, and so on. I would like to caution you that you must seek professional medical evaluation as soon as possible if you are not getting well from enteritis, and do not treat yourself to avoid aggravation of the condition, long-term inflammation leading to colon polyps, strictures and increased risk of cancer.