https://en.m.wikipedia.org/wiki/QT_interval

The QT interval is a measurement made on an electrocardiogram used to assess some of the electrical properties of the heart. (…) An abnormally long or abnormally short QT interval is associated with an increased risk of developing abnormal heart rhythms and sudden cardiac death.

QTc stands for QT corrected for heart rate, the medication that increases QTc will increase the risk of certain dangerous heart conditions.