INTRODUCTION:Adverse drug reactions (ADRs) occur frequently in hospitals and increase costs of health care; however, few studies have quantified the clinical and economic impact of ADRs in Colombia. OBJECTIVES:These impacts were evaluated by calculating costs associated with ADRs in patients hospitalized in the internal medicine ward of a Level 3 hospital located in Bogotá, Colombia. In addition, salient clinical features of ADRs were identified and characterized. MATERIAL AND METHODS:Intensive follow-ups for a cohort of patients were conducted for a five month period in order to detect ADRs; different ways to classify them, according to literature, were considered as well. Information was collected using the INVIMA reporting format, and causal probability was evaluated with the Naranjo algorithm. Direct costs were calculated from the perspective of payer, based on the following costs: additional hospital stay, medications, paraclinical tests, additional procedures, patient displacement to intermediate or intensive care units, and other costs. RESULTS:Of 836 patients admitted to the service, 268 adverse drug reactions were detected in 208 patients (incidence proportion 25.1%, occurence rate 0.32). About the ADRs found, 74.3% were classified as probable, 92.5% were type A, and 81.3% were moderate. The body system most often affected was the circulatory system (33.9%). Drugs acting on the blood were most frequently those ones associated with adverse reactions (37.6%). The costs resulting from medical care of adverse drug reactions varied from COL dollar 93,633,422 (USD dollar 35,014.92) to COL dollar 122,155,406 (USD dollar 45,680.94), according to insurance type, during the study period. CONCLUSIONS:Adverse drug reactions have a significant negative health and financial impact on patient welfare. Because of the substantial resources required for their medical care and the significant proportion of preventable adverse reactions, active programs of institutional pharmacovigilance are highly recommended.