Oral Magnesium Replacement
For mild to moderate hypomagnesemia, start with magnesium oxide 12 mmol (approximately 400 mg) at night, increasing to 12-24 mmol daily (400-800 mg) based on response and tolerance. 1, 2
Critical First Step: Correct Volume Depletion
Before initiating magnesium supplementation, you must correct sodium and water depletion with intravenous normal saline (2-4 L/day initially). 3, 1 This eliminates secondary hyperaldosteronism, which drives renal magnesium wasting and prevents effective oral repletion—supplementing magnesium without first correcting volume status will fail because ongoing renal losses will exceed supplementation. 1
Standard Oral Replacement Protocol
Dosing Recommendations
Magnesium oxide is the preferred oral formulation because it contains more elemental magnesium than other salts (each 400 mg tablet provides approximately 241 mg elemental magnesium). 2, 4
Initial dose: 12 mmol (400 mg) given at night when intestinal transit is slowest to maximize absorption. 1, 2
Titrate to 12-24 mmol daily (400-800 mg) depending on severity and response, typically given as 400 mg once or twice daily. 1, 2, 4
For patients with short bowel syndrome or significant malabsorption, higher doses up to 24 mmol daily may be required. 3, 1
Alternative Formulations
Organic magnesium salts (aspartate, citrate, lactate, glycinate) have superior bioavailability compared to magnesium oxide and cause fewer gastrointestinal side effects. 1, 2 Consider these alternatives when the goal is not treating constipation. 1
Divide supplementation into multiple doses throughout the day for continuous repletion and better tolerance. 1, 2
Absolute Contraindications
Never supplement magnesium if creatinine clearance is <20 mL/min—this creates life-threatening hypermagnesemia risk because kidneys are responsible for nearly all magnesium excretion. 1 Between 20-30 mL/min, avoid supplementation except in life-threatening emergencies (torsades de pointes) and only with close monitoring. 1
When Oral Therapy Fails
If oral supplementation doesn't normalize levels after 2-3 weeks:
Add oral 1-alpha hydroxy-cholecalciferol (0.25-9.00 μg daily) in gradually increasing doses to improve magnesium balance, but monitor serum calcium regularly to avoid hypercalcemia. 3, 1, 2
Consider intravenous or subcutaneous magnesium sulfate (4-12 mmol added to saline bags) for patients with short bowel syndrome, high-output stomas, or severe malabsorption where oral therapy is ineffective. 3, 1
Monitoring Timeline
Baseline: Check serum magnesium, potassium, calcium, and renal function; assess volume status. 1
2-3 weeks after starting: Recheck magnesium level and assess for side effects (diarrhea, abdominal distension). 1
Every 3 months: Monitor magnesium levels quarterly once dose is stable. 1 More frequent monitoring is needed if high GI losses, renal disease, or medications affecting magnesium are present. 1
Critical Pitfalls to Avoid
Never attempt to correct hypokalemia before normalizing magnesium—hypomagnesemia causes dysfunction of multiple potassium transport systems and increases renal potassium excretion, making hypokalemia resistant to potassium treatment until magnesium is corrected. 3, 1
Don't assume normal serum magnesium excludes deficiency—less than 1% of total body magnesium is in blood, so normal levels can coexist with significant intracellular depletion. 1
Most magnesium salts are poorly absorbed and may paradoxically worsen diarrhea or stomal output in patients with gastrointestinal disorders, so start low and titrate slowly. 3, 1, 2
Always check renal function before initiating supplementation—failing to identify renal impairment can lead to life-threatening hypermagnesemia. 1, 5
Target Levels and Expected Response
Target serum magnesium: >0.6 mmol/L (>1.8 mg/dL) with normalization typically within 2-3 weeks of adequate supplementation. 1, 2
Onset of action for oral magnesium is 7 hours to several days, with symptom resolution (muscle cramps, tetany, fatigue) expected within 24-72 hours after repletion begins. 1