This study evaluated the relationship between serum magnesium levels and QTc interval in over 8,500 patients admitted to the cardiac care unit. The results showed a dose-dependent relationship between higher magnesium levels and prolonged QTc interval. Specifically, magnesium levels of 2.4 mg/dL or higher were independently associated with a 2% increased risk of a prolonged QTc, while lower levels between 1.8-2.0 mg/dL were protective. This finding suggests that hypomagnesemia may protect against QTc prolongation, contrary to previous beliefs about magnesium's role, and warrants further experimental study of the molecular mechanisms involved.