What magnesium replacement should be given to an adult with low serum magnesium?

Treatment of Hypomagnesemia in Adults

First-Line Oral Magnesium Replacement

For mild to moderate hypomagnesemia, start with oral magnesium oxide 12 mmol (approximately 480 mg elemental magnesium) given at night, increasing to 24 mmol daily if needed after 1-2 weeks. 1

Critical First Step: Correct Volume Depletion

Before initiating any magnesium supplementation, you must address volume status:

• Administer IV normal saline (2-4 L/day initially) to restore sodium and water balance in patients with gastrointestinal losses, high-output stomas, or diarrhea. 1, 2
• Volume depletion triggers secondary hyperaldosteronism, which increases renal magnesium wasting and renders oral supplementation ineffective. 1, 2
• This is the most common reason oral magnesium therapy fails—never skip this step. 1, 2

Oral Magnesium Dosing Protocol

• Magnesium oxide 12 mmol at night (single dose) is the preferred first-line agent. 1
• Night-time administration exploits slower intestinal transit during sleep for maximal absorption. 1, 3
• If serum magnesium remains low after 1-2 weeks, escalate to 24 mmol daily (single or divided doses). 1
• Magnesium oxide provides the highest elemental magnesium content and is converted to magnesium chloride in gastric acid. 1

Alternative Oral Formulations

• For patients who cannot tolerate magnesium oxide due to diarrhea, use organic magnesium salts (aspartate, citrate, lactate, or glycinate) which have higher bioavailability and cause fewer GI side effects. 1, 3, 4
• These organic salts are better absorbed but still may worsen diarrhea in patients with malabsorption syndromes. 1

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Intravenous Magnesium Replacement

Severe Symptomatic Hypomagnesemia

For severe hypomagnesemia (serum Mg <0.50 mmol/L or <1.2 mEq/L) with symptoms, give 1-2 g magnesium sulfate IV over 15 minutes, followed by continuous infusion or repeated doses. 1, 2, 5

Life-Threatening Presentations

For torsades de pointes, ventricular arrhythmias, seizures, or cardiac arrest, administer 1-2 g magnesium sulfate IV bolus over 5 minutes immediately, regardless of the baseline serum magnesium level. 1, 2, 5

• This is a Class I recommendation from the American Heart Association. 1
• Follow the bolus with continuous infusion of 1-4 mg/min if needed for ongoing arrhythmias. 1

IV Dosing for Moderate Deficiency

• For mild deficiency: 1 g (8.12 mEq) IM every 6 hours for 4 doses. 5
• For severe hypomagnesemia: up to 250 mg/kg (approximately 2 mEq/kg) IM within 4 hours if necessary. 5
• Alternatively: 5 g (40 mEq) added to 1 L of D5W or normal saline for slow IV infusion over 3 hours. 5
• The rate of IV injection should generally not exceed 150 mg/minute (1.5 mL of 10% solution). 5

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Management of Refractory Cases

When Oral Therapy Fails

If oral magnesium supplementation does not normalize levels after 1-2 weeks at maximum dose (24 mmol daily), add oral 1-alpha hydroxy-cholecalciferol starting at 0.25 μg daily and titrating up to 9 μg. 1, 3, 2

• Monitor serum calcium weekly to avoid hypercalcemia. 1, 3
• For patients with short bowel syndrome or severe malabsorption, consider subcutaneous magnesium sulfate (4-12 mmol added to saline bags) 1-3 times weekly. 1, 3

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Critical Electrolyte Interactions

Correct Magnesium BEFORE Potassium or Calcium

Never attempt to correct hypokalemia or hypocalcemia before normalizing magnesium—these abnormalities are refractory to supplementation until magnesium is repleted. 1, 2

• Hypomagnesemia causes dysfunction of multiple potassium transport systems and increases renal potassium excretion. 1, 3
• Hypomagnesemia impairs parathyroid hormone release, causing refractory hypocalcemia. 1
• Calcium normalization typically occurs within 24-72 hours after magnesium repletion begins. 1

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Monitoring and Follow-Up

• Check serum magnesium 2-3 weeks after starting oral supplementation or after any dose adjustment. 1, 2
• Target serum magnesium within normal range (1.8-2.2 mEq/L or 0.70-1.10 mmol/L). 1, 2
• Once stable, monitor every 3 months. 1
• Always check and correct concurrent potassium, calcium, and phosphate abnormalities. 1, 2

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Critical Contraindications and Precautions

Renal Function Assessment

Magnesium supplementation is absolutely contraindicated when creatinine clearance is <20 mL/min due to risk of life-threatening hypermagnesemia. 1, 3

• Use extreme caution and reduced doses when CrCl is 20-30 mL/min. 1
• In severe renal insufficiency, maximum dose is 20 g/48 hours with frequent serum monitoring. 1, 5

Signs of Magnesium Toxicity

Monitor for loss of patellar reflexes, respiratory depression, hypotension, and bradycardia during IV replacement. 1, 5

• Have calcium chloride available to reverse magnesium toxicity if needed. 1

Pregnancy Considerations

Continuous maternal administration of magnesium sulfate beyond 5-7 days can cause fetal abnormalities. 5

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Common Clinical Pitfalls

• Failing to correct volume depletion first allows secondary hyperaldosteronism to perpetuate renal magnesium wasting despite supplementation. 1, 2
• Most magnesium salts are poorly absorbed and may paradoxically worsen diarrhea in patients with GI disorders—start low and titrate slowly. 1, 3
• Assuming normal serum magnesium excludes deficiency—less than 1% of total body magnesium is in blood, so normal levels can coexist with significant intracellular depletion. 6
• Never give bolus potassium for cardiac arrest—this is a Class III recommendation. 1