Magnesium (Mg) is the fourth most abundant cation in the body. An average adult contains approximately 25g (2000 mEq) of Mg.^1,2 Of the body’s Mg, the vast majory of this ion is in the intracellular compartment (99%), and the remaining 1% is in the extracellular fluid.³ The principal site of intracellular Mg is bone (60%–65%) where two distinct pools, cortical and trabecular, have been described. It is thought that Mg forms a surface constituent of the hydroxyapatite mineral component. Initially, much of this magnesium is readily exchangeable with serum and therefore represents a moderately accessible magnesium store, which can be drawn on in times of defi ciency. Approximately 25% to 30% is localized within the skeletal muscle, and about 10% to 15% in other non-muscle soft tissues.^4,5