Magnesium Dosage Recommendations
For general supplementation, the recommended daily allowance is 320 mg/day for women and 420 mg/day for men, with a tolerable upper limit of 350 mg/day from supplements to avoid gastrointestinal side effects, though this limit may be conservative based on recent evidence. 1
General Supplementation Dosing
Recommended Daily Allowance (RDA):
Tolerable Upper Intake Level:
- The established upper limit is 350 mg/day from supplemental sources 1, 2
- However, recent evidence suggests doses up to 520-1200 mg/day may be well-tolerated without significant diarrhea in many patients 3
- Diarrhea remains the primary dose-limiting side effect 2
Condition-Specific Dosing
Chronic Idiopathic Constipation
- Start with magnesium oxide 400-500 mg daily 1
- Titrate based on symptom response and tolerance 1
- Clinical trials have used up to 1.5 g/day 1
Short Bowel Syndrome
- Magnesium oxide 12-24 mmol daily (480-960 mg elemental magnesium) 1
- Administer at night when intestinal transit is slowest to maximize absorption 1
- Critical first step: Correct volume depletion with IV saline to address secondary hyperaldosteronism before supplementing magnesium 1
- If oral supplementation fails, consider IV or subcutaneous magnesium sulfate (4-12 mmol added to saline) 1
Erythromelalgia
- Start at RDA (350 mg daily for women; 420 mg daily for men) 1
- Increase gradually according to tolerance 1
- Liquid or dissolvable forms are better tolerated than pills 1
- For refractory cases: IV administration of 2g infused over 2 hours every 2-3 weeks 1
Hypomagnesemia Treatment
Mild Deficiency:
Severe Deficiency:
- Up to 250 mg/kg (approximately 2 mEq/kg) IM within 4 hours if necessary 4
- Alternatively: 5 g (40 mEq) added to 1 liter IV fluid infused over 3 hours 4
- For acute severe deficiency: 1-2 g IV over 15 minutes 1
Cardiac Emergencies
Torsades de Pointes:
- 25-50 mg/kg IV (maximum 2 g) as bolus for pulseless torsades 1
- Over 10-20 minutes for torsades with pulses 1
- Target magnesium level >2 mg/dL if QTc >500 ms 1
Refractory Status Asthmaticus:
- 25-50 mg/kg IV (maximum 2 g) over 15-30 minutes 1
Pre-eclampsia/Eclampsia
- Initial dose: 10-14 g total 4
- IV: 4-5 g in 250 mL fluid infused, or 4 g diluted to 10-20% concentration over 3-4 minutes 4
- Maintenance: 4-5 g IM into alternate buttocks every 4 hours, or 1-2 g/hour by continuous IV infusion 4
- Target serum level: 6 mg/100 mL for seizure control 4
- Maximum: 30-40 g per 24 hours 4
- Do not use beyond 5-7 days in pregnancy due to risk of fetal abnormalities 4
Critical Contraindications and Precautions
Absolute Contraindication:
- Creatinine clearance <20 mL/min 1, 2
- In severe renal insufficiency, maximum dose is 20 g/48 hours with frequent monitoring 4
Essential Monitoring for IV Administration:
- Continuous pulse, blood pressure, and ECG monitoring 2
- Hourly urine output 2
- Deep tendon reflexes 4
- Have calcium chloride immediately available as antidote 2
Formulation Considerations
Bioavailability by Type:
- Organic salts (citrate, malate, glycinate, acetyl taurate) have superior absorption compared to inorganic forms 1, 5
- Magnesium oxide is commonly used but has lower bioavailability 1
- Liquid or dissolvable forms are better tolerated than pills 1
Administration Rate:
- IV injection should generally not exceed 150 mg/minute 4
- Exception: Severe eclampsia with seizures may require faster administration 4
Common Pitfalls
- Never supplement magnesium without checking renal function first - toxicity develops rapidly in renal impairment 2
- Always correct volume depletion before magnesium supplementation in patients with diarrhea or high-output stomas, as hyperaldosteronism causes renal magnesium wasting 1
- Hypokalemia will be refractory to treatment until magnesium is normalized - always check and correct magnesium when treating hypokalemia 1
- Most magnesium salts may worsen diarrhea, limiting oral dosing 1, 2