Hypomagnesemia Replacement
For symptomatic or severe hypomagnesemia (serum Mg <0.5 mmol/L or <1.2 mg/dL), administer intravenous magnesium sulfate immediately; for mild, asymptomatic deficiency (0.5-0.7 mmol/L), use oral magnesium supplementation. 1
Critical First Step: Address Underlying Causes
Before initiating magnesium replacement, you must correct volume depletion and secondary hyperaldosteronism, as sodium and water depletion triggers aldosterone secretion that causes renal magnesium wasting—supplementation will fail if this is not addressed first. 2, 1 Administer intravenous saline to restore volume status, which reduces aldosterone and stops ongoing renal magnesium losses. 2
Additionally, identify and discontinue offending medications including diuretics, proton pump inhibitors, aminoglycosides, cisplatin, and cetuximab. 1
Route Selection Based on Severity
Life-Threatening Presentations
For torsades de pointes or life-threatening arrhythmias, give 1-2 g IV magnesium sulfate as a bolus push immediately. 1, 3
Severe Hypomagnesemia (Serum Mg <0.5 mmol/L or <1.2 mg/dL)
Intravenous magnesium sulfate is required for symptomatic or severe deficiency. 1, 3 The FDA-approved dosing for severe hypomagnesemia is up to 250 mg (approximately 2 mEq) per kg body weight IM within 4 hours if necessary, or alternatively 5 g (approximately 40 mEq) added to one liter of 5% dextrose or 0.9% saline for slow IV infusion over 3 hours. 3
For mild magnesium deficiency, the usual adult dose is 1 g (equivalent to 8.12 mEq) injected IM every 6 hours for 4 doses. 3
Critical safety point: The rate of IV injection should generally not exceed 150 mg/minute (1.5 mL of a 10% concentration). 3
Mild to Moderate Hypomagnesemia (0.5-0.7 mmol/L, Asymptomatic)
Oral magnesium oxide 12-24 mmol daily (approximately 480-960 mg elemental magnesium) is the preferred treatment. 2, 1 Administer at night when intestinal transit is slowest to maximize absorption. 2, 1
For general supplementation, start with the recommended daily allowance of 320 mg/day for women and 420 mg/day for men. 2
Special Clinical Scenarios
Refractory Hypokalemia
Always suspect and correct hypomagnesemia first when treating refractory hypokalemia, as magnesium deficiency causes dysfunction of potassium transport systems and increases renal potassium excretion—potassium supplementation will not work until magnesium is normalized. 2 Correct volume status with IV saline first, then normalize magnesium before expecting potassium supplementation to be effective. 2
Patients on Continuous Renal Replacement Therapy
Use dialysis solutions containing magnesium to prevent hypomagnesemia rather than relying on IV supplementation, as 60-65% of critically ill patients on continuous renal replacement therapy develop hypomagnesemia. 2, 1
Short Bowel Syndrome or High-Output Stomas
These patients require higher doses (12-24 mmol daily) due to significant gastrointestinal losses. 2 Rehydration with IV saline is the crucial first step to correct secondary hyperaldosteronism before magnesium supplementation. 2 If oral supplements fail to normalize levels, consider subcutaneous administration with 4 mmol magnesium sulfate added to saline, or oral 1-alpha hydroxy-cholecalciferol (0.25-9.00 μg daily) with careful monitoring of serum calcium. 2
Total Parenteral Nutrition
Maintenance requirements range from 8-24 mEq (1-3 g) daily for adults and 2-10 mEq (0.25-1.25 g) daily for infants. 3
Monitoring and Safety Considerations
Check renal function before any magnesium supplementation—avoid if creatinine clearance is <20 mL/min due to hypermagnesemia risk. 2 In severe renal insufficiency, the maximum dosage is 20 grams per 48 hours with frequent serum magnesium monitoring. 3
Continuous maternal administration beyond 5-7 days in pregnancy can cause fetal abnormalities. 3
Monitor for concurrent electrolyte abnormalities, as hypomagnesemia often coexists with hypocalcemia and hypokalemia, which will not correct until magnesium is repleted. 1
For patients with QTc prolongation >500 ms, replete magnesium to >2 mg/dL regardless of baseline level as an anti-torsadogenic countermeasure. 2
Common Pitfalls
Most oral magnesium salts are poorly absorbed and may worsen diarrhea or stomal output in patients with gastrointestinal disorders. 2 Organic magnesium salts (aspartate, citrate, lactate) have better bioavailability than magnesium oxide or hydroxide. 2
Attempting to correct magnesium without first addressing volume depletion and hyperaldosteronism will fail, as ongoing renal losses will exceed supplementation. 2
Do not exceed the tolerable upper intake level of 350 mg/day from supplements for general health maintenance to avoid adverse effects including diarrhea and gastrointestinal intolerance. 2