Background: Hydroxychloroquine has recently received attention as a treatment for COVID-19. However, hydroxychloroquine may prolong the QTc interval, thus increasing the risk of life-threatening arrhythmia. Many patients with COVID-19 have comorbidities, necessitating the use of several drugs simultaneously with hydroxychloroquine. However, the risk of QT prolongation due to drug-drug interactions (DDIs) between hydroxychloroquine and these co-medications has not been identified. Therefore, it is necessary to investigate the risk of QT interval prolongation due to DDIs between hydroxychloroquine and frequently used concurrent drugs.Methods and Results: Using 447,632 patients and 1,040,752 electrocardiograms, we investigated the risk of QT prolongation due to DDIs between hydroxychloroquine and 118 concurrent drugs frequently used in real-world practice. In the analysis, we observed that 11 drugs (trimebutine, tacrolimus, tramadol, rosuvastatin, ciclosporin, sulfasalazine, rofecoxib, diltiazem, piperacillin/tazobactam, and isoniazid) show DDIs with hydroxychloroquine in the direction of QT prolongation.Conclusions: We found 11 drugs that show significant (p <0.05) DDIs with hydroxychloroquine, thereby increasing the risk of QT prolongation in patients. It is necessary to consider prescribing alternative drugs that have less DDI when these drugs are concurrently administered with hydroxychloroquine. Further investigation is needed to assess more profoundly the risk of QT prolongation due to DDI with hydroxychloroquine of each drug that we found in this analysis.