1000 to 2000 mg (10 to 20 mL) IV one time at a rate not to exceed 0.5 to 2 mL/min. This dose may be repeated as necessary in severe cases of hypermagnesemia (where discontinuation of exogenous magnesium is inadequate) to temporarily reverse many of the toxic effects of magnesium in the central nervous system.
Furosemide (Lasix) may promote excretion of magnesium. It increases excretion of water by interfering with the chloride-binding cotransport system, which in turn inhibits sodium and chloride reabsorption in the ascending loop of Henle and distal renal tubule.
For these reasons, oral replacement should be given in the asymptomatic patient, preferably with a sustained-release preparation, given the ability of magnesium to induce diarrhea. Bioavailability of oral preparations is assumed to be 33% in the absence of intestinal malabsorption. Several preparations are available: Mag-Ox 400, containing magnesium oxide; Slow-Mag, containing magnesium chloride; and Mag-Tab, containing magnesium lactate. These preparations provide 5-7 mEq (2.5-3.5 mmol or 60-84 mg) of magnesium per tablet. Six to 8 tablets should be taken daily in divided doses for severe magnesium depletion. Two to 4 tablets may be sufficient for mild, asymptomatic disease. Mag-Ox 400 contains 242 mg (20 mEq) of elemental magnesium, but absorption is less efficacious.