OBJECTIVES: Was to determine the percentage of drug interactions' risk in elderly population receiving three or more drugs at the same time, also and degree of risk of severe, moderate and mild interactions. METHODS: This study was retrospectively-observational. The study included 50 patients of both genders, aged at least 65 years, who were treated in the first half 2012th in the Health Center Novi Sad which prescribed three or more drugs. Three sources of information about possible drug interactions were used: www.drugs.com, British National Formulary (BNF) and SPC of drug. RESULTS: The study included a total of 50 patients with mean age of 73.62 years. According to the number of possible drug interactions, most of them were from BNF (n = 204 or 45.43% compared to the number of possible drug combinations), then www.drugs.com (n = 203 or 45.21%) and from SPC source were significantly less (n = 150 or 33.40%). In relation to the degree of relevancy of drug interactions, according to the source www.drugs.com, the incidence of serious interactions was 3%, 29% moderate and 13% mild. Out of 50 patients analyzed, only in 9 patients (18%) were found complete compatibility in the number of interactions in all three data sources. A significantly higher number of interactions were at the level of pharmacodynamic (167 or 77.68%). CONCLUSIONS: With the increasing number of taken drugs at the same time, increases the total number of interactions with a correlation coefficient of 0.71. Publications and Internet could provide useful information of drug interactions, but we could not say what else is important in a particular patient. This research was supported by Provincial Secretariat for Science and Technological Development, Autonomous Province of Vojvodina project No 114-451-2458/2011 and by Ministry of Science and Technological Development, Republic of Serbia project No 41012.