Magnesium Salts
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Electrolyte replacement, Antacid (hydroxide), Osmotic laxative (citrate, hydroxide), Antiarrhythmic (IV sulfate, torsades)
Magnesium (oxide, citrate, sulfate, hydroxide, gluconate, chloride salts)
Mag-Ox, Slow-Mag, MagCitrate, Milk of Magnesia (hydroxide); IV sulfate generic
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Summary
Classes
Common uses
Hypomagnesemia replacement0, Constipation (citrate, hydroxide)0, Dyspepsia/heartburn (hydroxide antacid)0, Eclampsia and preeclampsia seizure prophylaxis (IV sulfate)0, Torsades de pointes (IV sulfate)0, Severe asthma exacerbation (IV sulfate)0, Migraine prophylaxis (oral, modest)0
Pharmacy
Starting dose
Replacement oxide 400-800 mg/d in divided doses (high diarrhea rate); citrate 200-400 mg/d (better tolerated, better absorbed); IV sulfate 1-2 g over 5-60 minutes for hypomagnesemia or torsades; eclampsia 4-6 g IV loading then 1-2 g/h
Preparations
Oxide 400, 500 mg tablets (240, 300 mg elemental); citrate 100, 150, 200 mg tablets; hydroxide oral suspension 400 mg/5 mL; sulfate IV 500 mg/mL ampules
US FDA Max
Indication-specific; renal clearance limits tolerable cumulative dosing
Pharmacology
Routes
Oral, IV
Onset
Oral: hours; IV: minutes
Duration
Variable
Half-life
Not meaningfully described (electrolyte; renally cleared)
Bioavailability
Highly salt-dependent: citrate ~25-30%; oxide ~4% (limited and causes osmotic diarrhea); chloride ~12%
Pregnancy
IV sulfate is the cornerstone of eclampsia/preeclampsia management; oral replacement also safe.[citation needed]
Legal status
OTC (oral salts) and Rx-only (IV) in US
Purported mechanism
Magnesium is essential for ~300 enzyme reactions including ATP-dependent processes (ATP-Mg complex is the bioactive form), competes with calcium at L-type voltage-gated channels (membrane stabilization, the basis of IV use in torsades and eclampsia seizure prevention), and modulates NMDA glutamate receptor activity.0 Renally cleared; accumulation in advanced CKD can produce neuromuscular and cardiac depression. Hypomagnesemia frequently co-exists with hypokalemia and is often the reason refractory potassium loss does not correct until magnesium is repleted.