Gastrointestinal toxicity arising from the use of 5-fluorouracil in the form of mucositis in patients with malignant neoplasms of the colon and rectosigmoid deserves close attention. An analysis of the literature allows us to conclude that the risk of damage to the mucous membrane of the stomach and duodenum during the use of chemotherapy with the inclusion of 5-fluorouracil is very high and amounts to 30-40%. In addition, there are many factors that can complicate the course of this undesirable phenomenon. These include the advanced age of patients, the presence of serious comorbidity and postoperative complications in patients, as well as infection with Helicobacter pylori. The lack of generally accepted recommendations for the prevention and treatment of damage to the lining of the stomach and duodenum during the use of chemotherapy is a serious problem in modern medical practice.