Magnesium Supplementation Dosing for Mild-to-Moderate Hypomagnesemia
For an adult with mild-to-moderate hypomagnesemia and normal renal function, start with 1 tablet (400 mg magnesium oxide ≈ 240 mg elemental magnesium) twice daily, which provides approximately 12 mmol elemental magnesium per day, administered preferably at night when intestinal transit is slowest. 1
Initial Assessment Before Starting Supplementation
Before prescribing any magnesium, you must verify renal function—magnesium supplementation is absolutely contraindicated when creatinine clearance falls below 20 mL/min due to life-threatening hypermagnesemia risk. 2, 1 Between 20-30 mL/min creatinine clearance, avoid magnesium except in emergencies like torsades de pointes. 2
Check for volume depletion first. If your patient has diarrhea, high-output stoma, or other gastrointestinal losses, correct sodium and water depletion with IV saline before starting magnesium supplementation. 2, 1 Secondary hyperaldosteronism from volume depletion increases renal magnesium wasting and will prevent effective oral repletion—supplementation will fail if you skip this step. 2, 1
Obtain baseline serum magnesium, potassium, and calcium levels, as hypomagnesemia frequently coexists with hypokalemia and hypocalcemia. 1 Never attempt to correct hypokalemia or hypocalcemia before normalizing magnesium—these electrolyte abnormalities are refractory to treatment until magnesium is corrected. 2, 1
Specific Dosing Protocol
Starting Dose
Begin with magnesium oxide 400 mg (approximately 240 mg elemental magnesium or 12 mmol) once daily at bedtime. 2, 1 Night-time dosing maximizes absorption because intestinal transit is slowest during sleep. 2, 1
The FDA-approved dosing for magnesium oxide as a supplement is 1-2 tablets daily. 3 For mild-to-moderate deficiency, this translates to 12 mmol elemental magnesium initially. 1
Dose Escalation
If serum magnesium remains low after 2-3 weeks, increase to 400 mg twice daily (total 24 mmol elemental magnesium per day). 2, 1 This higher dose is appropriate for patients with ongoing losses or malabsorption. 2, 1
For chronic idiopathic constipation, the American Gastroenterological Association recommends starting at 400-500 mg daily and titrating up to 1.5 g/day (approximately 900 mg elemental magnesium) based on response. 2 However, for pure magnesium repletion without constipation as the goal, the 12-24 mmol range (480-960 mg elemental magnesium) is appropriate. 2, 1
Monitoring Timeline
Recheck magnesium levels 2-3 weeks after starting supplementation. 2, 1 Also assess for gastrointestinal side effects including diarrhea and abdominal distension, which are the most common adverse effects. 2
After any dose adjustment, recheck levels again in 2-3 weeks. 2 Once on a stable dose with normalized magnesium levels, monitor every 3 months. 2 More frequent monitoring is needed if your patient has high gastrointestinal losses, renal disease, or takes medications affecting magnesium (diuretics, proton pump inhibitors, calcineurin inhibitors). 2
When Oral Therapy Fails
If oral magnesium oxide at 24 mmol daily fails to normalize levels after 4-6 weeks, consider adding oral 1-alpha hydroxy-cholecalciferol starting at 0.25 μg daily and titrating up to 9 μg daily to improve magnesium balance. 2, 1 Monitor serum calcium weekly when using this approach to avoid hypercalcemia. 2, 1
For refractory cases despite maximal oral therapy, particularly in patients with short bowel syndrome or severe malabsorption, intravenous or subcutaneous magnesium sulfate (4-12 mmol added to saline bags) may be necessary 1-3 times weekly. 2, 1
Critical Pitfalls to Avoid
Never start magnesium supplementation in a volume-depleted patient without first correcting sodium and water balance with IV saline. 2, 1 Secondary hyperaldosteronism will cause continued renal magnesium wasting despite supplementation, and your therapy will fail. 2, 1
Do not assume normal serum magnesium excludes deficiency—less than 1% of total body magnesium is in blood, so normal serum levels can coexist with significant intracellular depletion. 2 If clinical suspicion is high despite normal serum levels, consider red blood cell magnesium measurement or a therapeutic trial. 2
Avoid giving magnesium and calcium or iron supplements together—they inhibit each other's absorption. 1 Separate by at least 2 hours. 1
Most magnesium salts are poorly absorbed and may paradoxically worsen diarrhea in patients with gastrointestinal disorders. 2, 1 If diarrhea becomes problematic, consider switching to organic magnesium salts (glycinate, citrate, aspartate) which have better bioavailability and cause fewer gastrointestinal side effects, though they provide less elemental magnesium per tablet. 2, 1
Check for concurrent hypokalemia and correct magnesium first—hypomagnesemia causes dysfunction of multiple potassium transport systems and increases renal potassium excretion, making hypokalemia resistant to potassium treatment alone. 2, 1 Potassium supplementation will only work effectively after magnesium is normalized. 2, 1