OBJECTIVES: Since December 2010, online computerized prospective DUR(pDUR) has been implemented in Korea. pDUR involves the review of each prescription before the medication is dispensed to the individual patient. The pDUR is performed electronically by Health Insurance Review & Assessment Service (HIRA), which is a Korean governmental agency, and then HIRA provides medical institutions and pharmacies with information that can be helpful to them in preventing potential drug problems such as drug/drug interactions or ingredient duplication. The aim of this study was to assess the impact of the Korean pDUR implementation on the rate of drug-drug interactions (DDIs) using claims data of HIRA. METHODS: A before-after comparison of the prevalence of DDIs was conducted, using HIRA administrative claims data from medical institutions from January 2010 to December 2011. In addition, a paired t-test was applied to examine the difference between the pre- and post-pDUR. The analysis unit was the prescription issued and main outcome measures were the rates of DDIs within- (control group) or between- physician encounters. RESULTS: The mean DDIs rates (pre-test and post-test) within patient visits were 0.29‰ and 0.22‰, respectively. The mean rates of DDIs between visits were 0.94‰ (before) and 0.80‰ (after).  As a result of the t-test, we found that DDIs rate between encounters decreased significantly (t=3.04, p=0.0026) after the implementation of pDUR, whereas there is no significant reduction within encounters (t=1.15, p=0.2518). With respect to the prevalence of DDIs between drug groups, the most dramatic reduction was occurred between HMG CoA reductase inhibitors and anti-fungal agents. CONCLUSIONS: It seems effective that giving a direct feedback to prescribers by a prospective DUR. Further research is needed to assess the impact of DUR to final outcomes such as hospitalization.