What is the recommended replacement dose of magnesium for deficiency?

Magnesium Replacement Dosing

For mild to moderate magnesium deficiency, administer oral magnesium oxide 12-24 mmol daily (approximately 480-960 mg elemental magnesium), preferably at night when intestinal transit is slowest, while for severe symptomatic hypomagnesemia, give 1-2 g magnesium sulfate IV over 15 minutes followed by continuous infusion. 1, 2, 3

Initial Assessment and Preparation

Before initiating magnesium replacement, you must address several critical factors:

• Check renal function first - avoid magnesium supplementation entirely if creatinine clearance is <20 mL/min due to life-threatening hypermagnesemia risk 1, 2
• Correct volume depletion with IV saline to eliminate secondary hyperaldosteronism, which increases renal magnesium wasting and will cause ongoing losses despite supplementation 1, 2
• Measure concurrent electrolytes - hypomagnesemia commonly coexists with hypokalemia and hypocalcemia, and potassium supplementation will fail until magnesium is corrected 1, 2

Oral Replacement Dosing (Mild to Moderate Deficiency)

For asymptomatic or mildly symptomatic patients:

• Start with magnesium oxide 12-24 mmol daily (equivalent to 480-960 mg elemental magnesium or approximately 1-2 g magnesium oxide) 1, 2
• Administer at night when intestinal transit is slowest to maximize absorption 1
• Divide doses throughout the day if gastrointestinal side effects occur 1
• Alternative formulations: Organic magnesium salts (citrate, lactate, aspartate) have better bioavailability than oxide but are more expensive 1

For general supplementation without severe deficiency, the recommended daily allowance is 320 mg for women and 420 mg for men 1

Parenteral Replacement Dosing (Severe or Symptomatic Deficiency)

For severe symptomatic hypomagnesemia:

• Initial bolus: 1-2 g magnesium sulfate IV over 15 minutes 2, 3
• Followed by: 5 g (approximately 40 mEq) added to 1 liter of D5W or normal saline infused over 3 hours 3
• Maintenance: 24-48 mEq magnesium per 24 hours for 3-5 days 4

For life-threatening emergencies (torsades de pointes, severe arrhythmias):

• Give 1-2 g magnesium sulfate IV bolus over 5 minutes regardless of measured serum level 2, 3
• Maximum infusion rate: Generally should not exceed 150 mg/minute except in severe eclampsia with seizures 3

For intramuscular administration:

• Mild deficiency: 1 g (8.12 mEq) IM every 6 hours for 4 doses 3
• Severe deficiency: Up to 250 mg/kg (approximately 2 mEq/kg) IM within 4 hours if necessary 3

Special Clinical Scenarios

Short bowel syndrome or high GI losses:

• Require higher doses of 12-24 mmol daily due to ongoing losses 1
• If oral supplementation fails, consider subcutaneous magnesium sulfate 4-12 mmol added to saline bags 1-3 times weekly 2
• May add oral 1-alpha hydroxy-cholecalciferol 0.25-9.00 μg daily if oral magnesium alone is ineffective, but monitor calcium closely 1, 2

Patients on continuous renal replacement therapy:

• Use dialysis solutions containing magnesium to prevent ongoing losses 1, 2
• Hypomagnesemia occurs in 60-65% of critically ill patients on CRRT, especially with citrate anticoagulation 1

Cardiac patients with QTc prolongation >500 ms:

• Replete magnesium to >2 mg/dL regardless of baseline level as an anti-torsadogenic measure 1

Monitoring Schedule

• Initial recheck: 2-3 weeks after starting supplementation or any dose adjustment 1
• Maintenance monitoring: Every 3 months once on stable dosing 1
• High-risk patients (short bowel syndrome, high GI losses, renal disease): Check every 2 weeks during first 3 months, then monthly 1
• After IV replacement for cardiac emergencies: Recheck within 24-48 hours 1

Critical Pitfalls to Avoid

• Never supplement magnesium without first correcting volume depletion - secondary hyperaldosteronism will cause continued renal magnesium wasting that exceeds supplementation 1, 2
• Do not attempt to correct hypokalemia or hypocalcemia before magnesium - these will be refractory until magnesium is normalized 1, 2
• Avoid rapid IV infusion - can cause hypotension and bradycardia; have calcium chloride available to reverse toxicity 1, 2
• Most magnesium salts worsen diarrhea in patients with GI disorders, potentially creating a vicious cycle of increased losses 1, 2
• Maximum dose in severe renal insufficiency: 20 grams/48 hours with frequent serum monitoring 1, 3
• Avoid prolonged use in pregnancy - continuous maternal administration beyond 5-7 days can cause fetal abnormalities 3

Signs of Magnesium Toxicity

Monitor for loss of patellar reflexes, respiratory depression, hypotension, and bradycardia during IV replacement 2, 3