Abstract Aim As the population continues to age, the occurrence of chronic illnesses and comorbidities that often necessitate the use of polypharmacy has been on the rise. Polypharmacy, among other factors that tend to coincide with chronic diseases such as obesity, impaired kidney and liver function, and older age, can increase the risk of medication errors (MEs). Our study aims to evaluate the prevalence of MEs in the Internal medicine, Cardiology, and Neurology departments at the secondary level university hospital. Methods We conducted a prospective observational study of 145 patients electronic or paper-based data of inpatient prescriptions and patients’ pharmacokinetic risk factors, such as an impairment of renal and/or hepatic function, weight, and age. Results All included patients collectively received 1252 prescribed drugs. The median (Q1; Q3) number of drugs per patient was 8 (7;10). At least one ME was identified in 133 out of the 145 patients, indicating a significantly higher prevalence than hypothesized (91.7% vs. 50%; p < 0.001). There was moderate, positive correlation between the quantity of prescribed drugs and the number of MEs, meaning that the more drugs are prescribed, the higher the number of identified MEs (Spearman’s rho = 0.428; p < 0.001). Conclusion These findings suggest that there is a need of continuous medication education activity for prescribing physicians, continuous evaluation of prescription appropriateness to objectively identify the MEs, and to contribute to more rational patient treatment.