Background & Aims: Pharmacological treatment of cancer is associated with serious adverse effects including drug-drug interaction (DDI). The purpose of this study was evaluating DDIs in cancer patients hospitalize in the Intensive Care Unit (ICU) and identifying the type and severity of DDI.

Materials & Methods: The study population was all patients with cancer admitted to the ICU of Imam Khomeini hospital during 21 March 2013 to 20 March 2014. Patients’ information were extracted from the medical records. Then the recording demographic features and prescribed drugs in the first 24 hours of hospitalization were extracted and were investigated. DDIs were classified in terms of severity to as mild, moderate and severe into five groups.

Results: Almost 101 patients were evaluated (63 male and 38 female). The mean age was 59.47±1.69 and the average prescribed medications was 5.23±0.23. Then, 162 DDIs were identified that 14.81% of them were major, 77.16% moderate and 8.03% minor in the point of severity.  In the basis of documentation 14.19% of DDIs were established. The most DDIs were in the age group 61-80 years and the most involved drugs were: ranitidine (79.16), cephalosporins (63.162), anticoagulants (24.162) and narcotic analgesics (18.162).

Conclusion: This study shows that a high percentage of DDIs occurs in the ICU. Therefore, we suggest treating patients in ICU relying on a teamwork in drug prescribing.