Drug interactions represent an important area of research in modern medicine. Warfarin, as the basic drug for treatment and prevention of deep venous thrombosis, pulmonary embolism and other diseases, due to its specific composition, biotransformation, metabolism, individual dosing and the risk of bleeding represents a permanent challenge. In the attempt to evaluate the influence of drug interactions on the International Normalized Ratio (INR), the computer program for screening drug interactions called Lexi- Interact was used during this research as the basic parameter for monitoring warfarin therapy. In this paper, the analysis of 339 patients whose warfarin therapy began during hospitalization was conducted. Based on the program, they were divided into the group with (223 patients), and the group without (116 patients) possible clinically significant interactions. On the average, the patients were concurrently taking 5.28 drugs (1-13 drugs per patient). The analysis of results shows that 65.8% of patients were exposed to clinically significant interactions (1-6 interactions per patient). Statistically significant difference between the two groups was recorded only in the number of drugs, apart from warfarin, the patients were treated with. The interactions did not have a statistically significant impact on the INR values on the day 7 of therapy or on the dose of warfarin on the same day. A significant influence of interactions was not recorded in the division of patients into subgroups with target INR between 2 and 3, and those above and under those values; or on the frequency of recorded side-effects. In conclusion, this research did not prove the influence of warfarin interactions with other drugs on the International Normalized Ratio, and therefore individual titration and monitoring remains the basis of its introduction into therapy.