About 20-25% of all persons and about 90% of all patients who are acutely hospitalized in internal medicine departments have multiple acute or chronic diseases. They are multimorbid. The encounter with multimorbid patients has become the most common situation in the health care system. Theoretically, multimorbidity results in an innumerable potential disease constellations. In addition, the likelihood of interactions between diseases (disease-disease interactions, DDI) and the complexity increases overproportionately with each additional disease. However, multimorbidity often occurs in typical diadic, triadic, or higher characteristic combinations, in "disease clusters", e. g., vascular risk factors, heart and lung diseases, Frailty and dementia, psychiatric and somatic disorders. Such combinations lead to a worsening of the overall prognosis. In addition, DDIs are often difficult to treat or are life-threatening. Examples of DDIs include the following: anticoagulation and simultaneous severe bleeding, pain treatment and hypertension or renal insufficiency, depression and reduced medication adherence, chronic obstructive pulmonary disease and depression, Frailty and neurodepressant drugs and frequent falls, and combined psychiatric and somatic disorders. Such DDIs are common. Nevertheless, there are few studies and clinical guidelines that address these issues. The care of multimorbid patients is, therefore, heavily reliant upon guidelines developed mostly for single diseases. However, multimorbidity and serious DDIs are usually not addressed in these. Clinical guidelines can thus inadvertently jeopardize the safety of persons suffering from multiple diseases. In addition, stressful dilemmas arise for physicians encountering DDIs because of difficult treatment decisions.
Keywords: Anticoagulants; Disease–disease interactions; Hemorrhage; Hypertension; Pain, drug therapy.