Objectives: To characterize adverse drug reactions that were reported to the pharma¬covigilance program of a high complexity hospital between 2009 and 2012 and identify factors associated to preventability. Methods: observational, transversal study based on reports of adverse reactions reported to the institutional pharmacovigilance program. Univariate analysis was performed for qualitative variables and the chi-square test of independence was used for comparisons. The estimated sample size was 84 with a power of 80%. Results: a total of 2,973 adverse drug reactions were reported in 2,531 patients with similar frequencies in both genders and a mean age of 41.7 years (SD 24.42). Systemic antimicrobials and analgesics were the most frequently reported therapeutic groups (28.76% and 19.44%, respectively). The skin and appendix was the most affected body system (47.1%). Moderate and severe reactions accounted for 88.7% of the reported reactions; 9.4% of reported cases were preventable. Significant association was found between preventability and high alert medications, allergy history, whether or not the reaction was considered the cause for hospital admission, and patient age. Conclusion: the results are comparable with other studies, and they helped to identify factors amenable to intervention to promote safe medication use. Identifying factors associated to preventability, some unreported to date, is an important finding in the search of strategies to minimize preventable errors.