Medication is one of the causes of QT prolongation, which is generally asymptomatic but can lead to fatal ventricular tachycardia/ fibrillation (VT/ VF), especially in the elderly. This study aimed to evaluate the prescribing patterns in the elderly with the risk of QT prolongation. A retrospective database study was conducted to reveal the prescribing patterns in ambulatory patients older than 65 years from 34 tertiary care hospitals in 2019. We found 3,569,139 prescriptions during the study period. 33.6% of them contained at least one drug with QT prolongation potential. The maximum number of drugs with QT prolongation potential found per prescription was 7. Omeprazole (15.63 %), furosemide (5.36%), and tramadol (3.57%) were the most frequently prescribed drugs with QT prolongation potential. The most frequently found drug interaction with the potential for QT prolongation was trazodone – fluoxetine. In addition, cardiovascular and antidiabetic drugs were the common concurrent medications found in this study, indicating that cardiovascular diseases and diabetes were the common risk factors for QT prolongation in the Thai elderly. Pharmacovigilance for the use of proton-pump inhibitors, diuretics, or the combination of antipsychotics and antidepressants on the risk of QT prolongation-related adverse events should be performed.