OBJECTIVES: To describe the frequency of adverse drugs events (ADEs) as possible causes of request of drugs not included in essential Medicines list in Colombia  METHODS: This was a retrospective, descriptive study developed in a private medical insurance company in Bogota, Colombia. Data were obtained from drug request form of drugs not included in a national essential Medicines list. We analyzed the content of the notes to identify the records related to the occurrence of ADEs in the period 2005 to 2008. Information concerning the adverse event and the drug involved was recorded in a data collection instrument developed by the researchers. The pharmacological classification of drugs was performed according to the Anatomical Therapeutic Chemical Classification System (ATC). Univariate descriptive statistical analysis was performed  RESULTS: A total of 116 cases of ADEs were detected. The level 1 groups of the ATC of drugs with greater frequency of ADEs were the cardiovascular agents (66; 47.15%), nervous system agents (34; 23.7 %) and antineoplastic and immunomodulating agents (21; 14.7 %). The great majority was cases of light severity (89; 62.7 %) and classified as possible (66; 48.4 %).  CONCLUSIONS: We conclude that our study encourages the private medical insurance companies in developing countries to design pharmacosurveillance programs; recognizing the importance of looking for new sources of report of adverse reactions to diminish the under-notification of ADEs.