Interaction between drugs constitutes a frequent cause of adverse drug reaction. The aim of this study, was to provide an analysis of potential drug interactions. The study groups were 225 patients from the dialysis unit and 519 patients from the heart failure centre. Discharge prescriptions from the two units were collected for six months during 2011. In the heart failure centre, 74% of patients received prescriptions for drug combinations that may have adverse interactions. The most frequent and important potential interactions concerned furosemide with enalapril,17%, and acetylsalicylic acid with metoprolol, 15%. In the dialysis unit, 19% of patients received prescriptions that may have adverse interactions. Two examples were algeldrate with amlodipine, 6%, In 1.3% of patients increase in INR occurred owing to interactions. The results have revealed particularly high risk for potential interactions in heart failure centre patients because of the prescription of many drugs and because of the types of these drugs.