Introduction: Drug drug interactions can effect the success of stem cell transplantation process. Therefore, analyzing the risk of these interactions would be helpful for practitioners. This study was conducted to identify drug-drug interactions in allogeneic and autologous stem cell transplantation patients before and after transplantation. 
 Methods: Patients who underwent allogeneic stem cell transplantation and autologous stem cell transplantation were included in the study. Patients’ treatment sheets were collected ten days before transplantation day, on the transplantation day, and ten days after transplantation day. Drug-drug interactions were analyzed by using four drug-drug interaction checking databases.
 Results: 50 patients from both transplantation types were included. The mean ages for allogeneic and autologous transplants were 42.4 and 51.8, respectively. 52% of allogeneic transplants and 28% of autologous transplants were on ≥5 drugs at tenth day before transplantation. The means of interactions in allogeneic and autologous stem cell transplantation patients were 75.42 and 43.62, respectively. The detection of at least one contraindicated interaction in allogeneic and autologous transplant patients were 94% and 92%, respectively. In 48% of allogeneic transplants and 36% of autologous transplants, two or more contraindicated interactions were detected.
 Conclusion: Allogeneic stem cell transplantation patients experienced almost two times more drug-drug interactions than autologous stem cell transplantation patients. Transplantation type, being in the pre and post-transplantation period and the drug number at the beginning of the transplantation process matter in terms of the number of drug-drug interactions. Identifying interactions in terms of transplantation type and existing medications is very important.
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