ObjectiveTo determine the frequency of adverse drug reactions in surgical intensive care units and evaluate their effect on the length of stay.DesignProspective cohort study. Between May 1997 and December 1999, while the patients were staying in the surgical intensive care unit, data were gathered regarding suspected adverse drug reactions and on different variables related to the length of stay.SettingSurgical intensive care units of our hospital.PatientsA total of 401 patients hospitalized in the surgical intensive care unit.Main ResultsIn 37 of the 401 patients seen (9.2%; 95% confidence interval, 6.6–12.5), 39 different adverse drug reactions were detected. The adverse drug reactions were most frequently caused by the following drugs: morphine hydrochloride (n = 13), meperidine hydrochloride (n = 9), and metamizole (n = 7). Five adverse drug reactions were severe, the suspected medication had to be discontinued in 14 cases, and new drugs were necessary to manage the adverse drug reaction in 28 cases. The crude estimation of the effect of adverse drug reactions performed on the length of stay with a bivariant regression model indicated that each adverse drug reaction was related to an increase of 3.39 days (95% confidence interval, 1.47–5.31) in the length of stay. This estimation was reduced to 2.31 days (95% confidence interval, 0.64–3.99) when considering other variables that might cause confusion for analysis, although it is still important.ConclusionsAdverse drug reactions are a significant clinical and economic problem in surgical intensive care units.