Hypermagnesemia is defined as a magnesium serum level greater than 2.7 mEq/L. The increased level occurs in the extracellular compartment and results from an increased intake or decreased excretion of magnesium. Though hypermagnesemia rarely occurs, an elevated level may result from an excessive intake of magnesium through magnesium-containing antacids or cathartics. It also may happen through an excessive administration of intravenous magnesium sulfate, particularly in the treatment of toxemia during pregnancy. Hypermagnesemia may also occur with the use of lithium (Figure 24-1).