Treatment of Hypomagnesemia
For mild hypomagnesemia, start with oral magnesium oxide 12-24 mmol daily (preferably given at night), but first correct any sodium and water depletion to address secondary hyperaldosteronism that perpetuates magnesium wasting. 1
Initial Assessment and Correction of Underlying Factors
Before initiating magnesium supplementation, you must address volume status:
- Correct water and sodium depletion first using intravenous saline if the patient has high output stoma, diarrhea, or other fluid losses 2, 1
- This step is critical because sodium depletion triggers secondary hyperaldosteronism, which increases renal magnesium excretion and creates a vicious cycle where supplementation fails 1
- Check renal function before any magnesium therapy—avoid supplementation if creatinine clearance is <20 mL/min due to hypermagnesemia risk 1
Oral Magnesium Therapy for Mild-Moderate Deficiency
Magnesium oxide is the preferred oral formulation because it contains more elemental magnesium than other salts and converts to magnesium chloride in the stomach 1:
- Start with 12 mmol magnesium oxide at night when intestinal transit is slowest to maximize absorption 2, 1
- Increase to 24 mmol daily if needed based on response 2, 1
- Alternative: Organic magnesium salts (aspartate, citrate, lactate) have higher bioavailability than magnesium oxide and can be considered if gastrointestinal side effects are problematic 1
Common pitfall: Most magnesium salts are poorly absorbed and may worsen diarrhea or stomal output in patients with gastrointestinal disorders 1. If this occurs, consider switching formulations or moving to parenteral therapy.
Parenteral Magnesium for Severe or Symptomatic Deficiency
For severe hypomagnesemia (<1.2 mg/dL or 0.5 mmol/L) or symptomatic patients, use IV magnesium sulfate 3, 4:
- Mild deficiency: 1 g (8.12 mEq) IM every 6 hours for 4 doses 3
- Severe hypomagnesemia: Up to 250 mg/kg (approximately 2 mEq/kg) IM over 4 hours, or 5 g (40 mEq) added to 1 liter of IV fluid infused over 3 hours 3
- Rate of IV injection should not exceed 150 mg/minute except in life-threatening situations 3
For cardiac arrhythmias associated with hypomagnesemia (including torsades de pointes), give 1-2 g IV magnesium bolus regardless of measured serum levels 2, 1
Refractory Cases and Alternative Approaches
If oral magnesium fails to normalize levels after adequate trial:
- Add oral 1-alpha hydroxy-cholecalciferol in gradually increasing doses (0.25-9.00 μg daily) to improve magnesium balance 2, 1
- Monitor serum calcium regularly to avoid hypercalcemia 2, 1
- Subcutaneous magnesium sulfate (4 mmol added to saline) can be given 1-3 times weekly for patients requiring chronic supplementation 2, 1
Special Considerations
In patients with refractory hypokalemia, always suspect concurrent hypomagnesemia 2, 1:
- Magnesium deficiency causes dysfunction of potassium transport systems and increases renal potassium excretion 2
- Hypokalemia will be resistant to potassium treatment until magnesium is corrected 2, 1
- Normalize magnesium first, then potassium supplementation will be effective 1
For short bowel syndrome or high-output stoma patients 2:
- These patients have particularly high magnesium losses (jejunostomy fluid contains significant magnesium) 2
- May require higher doses (12-24 mmol daily) and often need long-term supplementation 2, 1
- Reduce dietary lipids to improve magnesium absorption 1
Target serum magnesium level: Aim for >0.6 mmol/L (>1.5 mg/dL), with normal range being 1.8-2.2 mEq/L 1
Critical Pitfalls to Avoid
- Never give IV potassium bolus for cardiac arrest in suspected hypokalemia without first addressing magnesium 2
- Do not exceed 30-40 g magnesium sulfate per 24 hours in any patient 3
- In severe renal insufficiency, maximum dose is 20 g/48 hours with frequent serum monitoring 3
- Avoid prolonged IV magnesium in pregnancy beyond 5-7 days as it can cause fetal abnormalities 3
- Monitor for magnesium toxicity: hypotension, bradycardia, respiratory depression, loss of deep tendon reflexes 1