lmmunosuppressants are a relatively new group of medications with various chemical entities and indications for use ranging from prevention of transplanted organs to autoimmune and rheumatoid diseases. lt is beyond dispute that their use and indication spectrum will grow with time. Immunosuppressants currently registered in Croatia have 290 clinically significant drug interactions, 34.8 % of ali interactions are of the level of clinical significance D, 32.8 % level X and 32.4 % level C. Cyclosporine is the immunosuppressant with the largest number of drug interactions, while inhibitors of TNF-a, inhibitors of interleukin and antithymocyte globulin have the fewest interactions. lt is understandable that cyclosporine has most interactions since it has been in use and in the world market for the longest time and has the widest indications. Even though they have the fewest interactions, most interactions of TNF-a inhibitors are of level X compared to level C and D interactions. Regarding the ATC classification system, immunosuppressants mostly interact with drugs of groups J and L. These data are very important in clinical practice, especially during combined drug therapy. Almost 20 % of ali interactions are among immunosuppressants themselves. Based on their metabolism mechanism via CYP3A4 and P-glycoprotein, one would conclude that the mechanism of immunosuppressant interaction would mostly be of pharmacokinetic nature, but results show that more than 50% of interactions are of a still unknown mechanism. This can be attributed to the fact that immunosuppressants are quite new in the global drug market and their use with other medications is still insufficiently investigated.