Magnesium is an essential nutrient, meaning that your body needs it for healthy functioning. It is found in significant quantities throughout the body and used for numerous purposes, including muscle relaxation, blood clotting, and the manufacture of ATP (adenosine triphosphate, the body’s main energy molecule). It has been called nature’s calcium channel blocker. The idea refers to magnesium’s ability to block calcium from entering muscle and heart cells. A group of prescription heart medications work in a similar way, although much more powerfully. This may be the basis for some of magnesium’s effects when it is taken as a supplement in fairly high doses.
Source: Magnesium | Winchester Hospital