IV Magnesium Dosing for Hypomagnesemia
For severe symptomatic hypomagnesemia, administer 1-2 g magnesium sulfate IV bolus over 5-15 minutes, followed by continuous infusion if needed; for mild-moderate asymptomatic cases, use oral magnesium oxide 12-24 mmol daily as first-line therapy. 1, 2
Severity-Based Treatment Algorithm
Severe Symptomatic Hypomagnesemia (Mg <0.5 mmol/L or <1.2 mg/dL with symptoms)
Immediate IV therapy is required:
- Initial bolus: 1-2 g magnesium sulfate IV over 5-15 minutes 1, 2
- Followed by: Continuous infusion of 5 g (approximately 40 mEq) added to 1 liter of D5W or normal saline, infused over 3 hours 2
- Alternative regimen: 1 g (8.12 mEq) IM every 6 hours for 4 doses (total 32.5 mEq/24 hours) 2
- Maximum infusion rate: Do not exceed 150 mg/minute (1.5 mL of 10% solution) except in life-threatening arrhythmias 2
Life-Threatening Presentations (Torsades de Pointes, Cardiac Arrhythmias)
Give magnesium regardless of measured serum level:
- Dose: 1-2 g magnesium sulfate IV bolus over 5 minutes 1
- This applies even if baseline magnesium appears normal, as tissue depletion may exist despite normal serum levels 1
Mild-Moderate Hypomagnesemia (Mg 0.5-0.7 mmol/L or 1.2-1.7 mg/dL)
Oral therapy is first-line unless symptomatic:
- Magnesium oxide: 12 mmol at night initially, increase to 12-24 mmol daily based on response 1, 3
- Magnesium oxide is preferred as it contains more elemental magnesium than other salts 3
- Alternative: Organic salts (aspartate, citrate, lactate) have higher bioavailability and may be better tolerated 3
- Administer at night when intestinal transit is slowest to maximize absorption 3
Critical Pre-Treatment Steps
Before initiating magnesium replacement:
- Correct volume depletion first with IV saline to eliminate secondary hyperaldosteronism, which increases renal magnesium wasting 1
- Verify adequate renal function before any magnesium supplementation 4
- In severe renal insufficiency: Maximum dose is 20 g/48 hours with frequent serum monitoring 1, 2
Concurrent Electrolyte Management
Magnesium must be corrected before other electrolytes:
- Replace magnesium FIRST, then calcium and potassium - these will be refractory to treatment until magnesium is normalized 1
- Calcium normalization typically occurs within 24-72 hours after magnesium repletion begins 1
- Hypokalemia is resistant to potassium treatment alone when hypomagnesemia is present 1
Monitoring and Safety
Essential monitoring parameters:
- Watch for magnesium toxicity: Loss of patellar reflexes, respiratory depression, hypotension, bradycardia 1
- Target serum level: >0.6 mmol/L minimum, ideally 1.8-2.2 mEq/L (normal range) 3
- Have calcium chloride available to reverse magnesium toxicity if needed 1
- Monitor calcium, phosphorus, and potassium levels concurrently 1
Common Pitfalls to Avoid
Key clinical considerations:
- Rapid infusion causes hypotension and bradycardia - adhere to recommended infusion rates 1
- Do not mix magnesium sulfate with calcium or vasopressors in the same IV solution 1
- Use central venous access when possible to avoid tissue injury from extravasation 1
- Most oral magnesium salts worsen diarrhea in patients with GI disorders - consider parenteral route in these patients 1, 3
- Separate calcium and iron supplements by at least 2 hours from magnesium as they inhibit absorption 1
Special Populations
Patients with malabsorption or short bowel syndrome:
- May require higher oral doses or parenteral supplementation 1
- Consider subcutaneous magnesium sulfate 4-12 mmol in saline bags 1-3 times weekly for chronic management 1
- Add oral 1-alpha hydroxy-cholecalciferol (0.25-9.00 μg daily) in gradually increasing doses if refractory to standard therapy, monitoring calcium to avoid hypercalcemia 1
Pregnant patients: