Background: QT prolongation is a well-known complication when tramadol or zolpidem is ingested in large amounts acutely. However, the blood drug concentration resulting in QT prolongation or tachyarrhythmia when tramadol and zolpidem are ingested simultaneously in large amounts has not yet been reported. We report a case of acute intoxication by tramadol and zolpidem resulting in QT prolongation in a patient in whomserial blood and urine tramadol and zolpidemconcentrationswere determined.Case presentation:A 38-year-old male patient presented tothe emergency medical centrebecause ofpoisoning from 3 g of tramadol and 50 mg of zolpidemat 3 h before hisemergency department(ED) visit.During supportive treatment, he developed QT prolongation without clinical manifestations. He was discharged five days after admission without any sequelae. We measured the blood and urine concentrations of tramadol and zolpidemat various time points, which revealed ablood tramadol concentration-dependent change in QTc intervals and increasing blood tramadol concentration at 8 h after the ED visit,with a sustainedlow zolpidem concentration.Tramadol and zolpidem are metabolized by the same enzyme, cytochrome P450 3A4.Therefore, competitive inhibition bya P450 3A4isoenzyme may lead to increases indrugtoxicity. The QT interval in patients acutely intoxicated bytramadol should be evaluated carefully, particularlywhen tramadol is co-ingested with other drugs.Conclusions:Considering the half-life of tramadolor zolpidem and potential for continued absorption of drugs remaining in the gastrointestinal tract, it is necessary to observe patients viacardiac monitoring for more than 36 hafteracute intoxication.The blood concentration of tramadol may increaseand result in QT prolongationeven afterappropriate initial treatment.