Introduction. Adjuvant therapy with the inclusion of 5-fluorouracil is used in patients with neoplasms of the colon and rectosigmoid in the postoperative period. However, along with a positive antitumor effect, 5-fluorouracilcontaining regimens have a negative effect on the mucosa of the gastrointestinal tract, which often leads to the need to replace or cancel drugs, thereby reducing the effectiveness of treatment.Aim of the study was to improve the idea of the possibility of erosive and ulcerative lesions of the stomach and duodenum during treatment with 5-fluorouracil in patients with malignant tumors of the colon and rectosigmoid and the factors that aggravate their course, as well as to highlight the problem of prevention and treatment of gastrointestinal complications.Materials and methods The search for publications by keywords was carried out in the databases of PubMed, Google Scholar, elibrary.ru and Russian specialized journals related to therapy, surgery, gastroenterology, oncology for the period from 2010 to 2022.Results and Discussion 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.Conclusion Gastrointestinal toxicity associated with the use of 5-fluorouracil in the form of mucositis in patients with malignant neoplasms of the colon and rectosigmoid deserves close attention. The presence of damage to the mucous membrane of the stomach and duodenum in a patient requires immediate therapy, as this will improve the long-term results of the treatment of the underlying disease and ensure a high quality of life for the patient.