Ceftriaxone and lansoprazole are commonly prescribed medications. In a single-center retrospective cohort study of 380 000 patients, lansoprazole combined with ceftriaxone was associated with prolonged corrected QT (QTc) intervals. On average, QTc intervals were 12 milliseconds (95% CI, 7-15 milliseconds) longer in men and 9 milliseconds (95% CI, 5.2-11.3 milliseconds) longer in women who were receiving ceftriaxone and lansoprazole concurrently compared with patients receiving either drug alone.