We investigated the impact of Bazett (B) and Fridericia (F) correction formulas on the evaluation of QT prolongation in patients during methadone administration and to explore the relationship between heart rate and corrected QT interval (QTc) according to both correction formulas. This study was conducted as a single-center, retrospective observational study. Subjects were cancer patients who underwent electrocardiographic evaluation for methadone therapy at our institute from April 1, 2013, to August 31, 2023. The study assessed the incidence of QT prolongation and analyzed the correlation between heart rate and QTc derived from the B and F formulas. The mean QTc of 430.3±25.8 msec for the B formula and 409.2±20.8 msec for the F formula. The incidence of QT prolongation was significantly lower with the F formula (8.4%) compared to the B formula (27.7%), indicating a 19.3% reduction in QT prolongation cases (p<0.001). Additionally, the difference in QTc between the two formulas increased with an increase in heart rate (p<0.001). These results suggest that the F formula for QT interval correction in methadone therapy potentially expands the eligible patients for this therapy.