What is the recommended dosage of magnesium for an adult patient with hypomagnesemia?

Magnesium Dosing for Hypomagnesemia

For mild hypomagnesemia, give oral magnesium oxide 12-24 mmol daily (starting with 12 mmol at night); for severe or symptomatic hypomagnesemia, give 1-2 g IV magnesium sulfate over 15 minutes, followed by continuous infusion if needed. 1, 2

Treatment Algorithm Based on Severity

Mild Hypomagnesemia (Asymptomatic, Mg >1.2 mg/dL)

Oral magnesium oxide is first-line therapy at 12 mmol given at night, increasing to 24 mmol daily if needed. 1, 3

• Magnesium oxide contains more elemental magnesium than other salts and is converted to magnesium chloride in the stomach, making it the preferred oral formulation 3
• Administering at night when intestinal transit is slowest maximizes absorption 1, 3
• Before starting magnesium, correct water and sodium depletion with IV saline to eliminate secondary hyperaldosteronism, which increases renal magnesium wasting 1, 3
• Most magnesium salts are poorly absorbed and may worsen diarrhea or stomal output in patients with gastrointestinal disorders 1, 3

Severe Hypomagnesemia (Mg <1.2 mg/dL or Symptomatic)

Give 1 g magnesium sulfate (equivalent to 8.12 mEq) IM every 6 hours for 4 doses, or 5 g (approximately 40 mEq) added to 1 liter of IV fluid infused over 3 hours. 2

• For severe hypomagnesemia, as much as 250 mg/kg (approximately 2 mEq/kg) may be given IM within 4 hours if necessary 2
• The FDA label specifies that IV injection should generally not exceed 150 mg/minute (1.5 mL of 10% concentration) 2

Life-Threatening Presentations

For torsades de pointes, ventricular arrhythmias, or seizures, give 1-2 g magnesium sulfate IV bolus over 5 minutes regardless of baseline magnesium level. 4, 1

• This is a Class I recommendation from the American Heart Association for torsades de pointes with prolonged QT interval 4, 1
• Follow with continuous infusion of 1-4 mg/min if needed 4
• Monitor for magnesium toxicity including loss of patellar reflexes, respiratory depression, hypotension, and bradycardia 1

Critical Sequencing of Electrolyte Replacement

Always replace magnesium before attempting to correct hypocalcemia or hypokalemia, as these will be refractory to treatment until magnesium is normalized. 1

• Hypomagnesemia causes dysfunction of potassium transport systems and increases renal potassium excretion 1
• Calcium supplementation will be ineffective until magnesium is repleted, with calcium normalization typically occurring within 24-72 hours after magnesium repletion begins 1

Dosing Adjustments and Monitoring

Renal Insufficiency

In severe renal insufficiency, the maximum dose is 20 grams over 48 hours with frequent serum magnesium monitoring. 1, 2

• Establish adequate renal function before administering any magnesium supplementation 5
• For patients on continuous renal replacement therapy, use dialysis solutions containing magnesium to prevent ongoing electrolyte derangements 1

Maintenance Dosing

To maintain total serum magnesium above 2.0 mg/dL in high-risk patients, expect to administer 2 g IV magnesium sulfate at least twice daily. 6

• A 2024 study in 12,618 veterans showed that average total serum magnesium dropped below 2.0 mg/dL within 24 hours of administration, with less than half remaining above 2.0 mg/dL just 12 hours after a 2 g dose 6

Special Populations and Alternative Routes

Malabsorption or Short Bowel Syndrome

For patients failing oral therapy, consider subcutaneous magnesium sulfate 4-12 mmol added to saline bags 1-3 times weekly. 1

• A 2019 case series demonstrated that subcutaneous magnesium sulfate was effective and safe for chronic hypomagnesemia management 7
• Initially use IV magnesium sulfate, then transition to oral magnesium oxide and/or 1-alpha cholecalciferol 1, 3

Refractory Cases

If oral magnesium fails, add oral 1-alpha hydroxy-cholecalciferol 0.25-9.00 μg daily in gradually increasing doses to improve magnesium balance, monitoring serum calcium regularly to avoid hypercalcemia. 1

Common Pitfalls

• Never administer calcium and magnesium supplements together—they inhibit each other's absorption; separate by at least 2 hours 1
• Do not mix magnesium sulfate with vasopressors or calcium in the same IV solution 1
• Rapid infusion can cause hypotension and bradycardia; use a central venous catheter for administration to avoid tissue injury from extravasation 1
• Continuous maternal administration beyond 5-7 days in pregnancy can cause fetal abnormalities 2
• Have calcium chloride available to reverse magnesium toxicity if needed 1