At present, comorbidity is one of the current interdisciplinary problems of clinical medicine. At the same time, in the modern literature the number of teaching and study guides, monographs and other scientific publications reflecting the essence of comorbidity and its prognostic significance for the patient, is clearly inadequate. Also, no national clinical guidelines exist for managing patients with comorbid diseases. The aim of the review is to draw practical physicians’ attention to the importance of timely detection of comorbid nature of the internal disease necessary for the correct choice of therapeutic measures for patients suffering from multiple diseases, early prevention of possible complications and the formation of a favorable prognosis. Symptomatic gastroduodenal ulcer lesions were chosen as one of the examples of comorbid pathology, late diagnosis and absence of early prevention of which can become the causes of gastrointestinal bleeding and related deaths in patients. The article reflects the definition of comorbidity, existing synonyms, its main types, and methods of evaluation. The causes and factors of the development of comorbid diseases, their prevalence in the population are analyzed. Attention is focused on the rules for formulation of clinical diagnosis for a comorbid patient, the need to take into account comorbidity when choosing an algorithm for integrated diagnosis and rational treatment of the disease. The importance of assessing the risk of drug interactions in the pharmacotherapy of comorbid diseases and possible complications are emphasized. The epidemiology of symptomatic gastroduodenal ulcers, their pathogenesis, common clinical presentation and its features depending on a specific type, complexity of diagnosis are analyzed in detail.