Correcting Serum Magnesium Deficit
For mild hypomagnesemia, start oral magnesium oxide 12–24 mmol daily (480–960 mg elemental magnesium) taken at night; for severe or symptomatic cases (cardiac arrhythmias, seizures, tetany), give 1–2 g IV magnesium sulfate over 5–15 minutes immediately, regardless of measured serum level. 1
Critical First Step: Correct Volume Depletion Before Magnesium
Before administering any magnesium, you must correct sodium and water depletion with IV normal saline (2–4 L/day initially) to eliminate secondary hyperaldosteronism—otherwise renal magnesium wasting will continue and oral supplementation will fail. 1, 2 This is the single most common pitfall: starting magnesium in volume-depleted patients with diarrhea, high-output stomas, or gastrointestinal losses allows aldosterone to drive ongoing renal magnesium excretion despite supplementation. 1, 2
Volume depletion triggers aldosterone secretion, which increases renal sodium retention at the expense of magnesium and potassium, creating high urinary losses despite total body depletion. 2 Check for orthostatic vital signs and measure urinary sodium—levels <10 mEq/L confirm volume depletion with secondary hyperaldosteronism. 2
Assess Severity and Choose Route
Life-Threatening Presentations (IV Magnesium Required)
Give 1–2 g magnesium sulfate IV bolus over 5 minutes immediately for: 1
- Torsades de pointes or polymorphic ventricular tachycardia 1
- Cardiac arrest 1
- Seizures or tetany 1
- Ventricular arrhythmias 1
Administer this dose regardless of baseline serum magnesium level—total body stores may be severely depleted even with "normal" serum values, since less than 1% of total body magnesium is in blood. 1, 3, 4
For severe symptomatic hypomagnesemia (serum Mg <0.50 mmol/L or <1.2 mg/dL) without immediate life threat, give 1–2 g IV magnesium sulfate over 15 minutes, followed by continuous infusion of 1–4 mg/min or 4–5 g in 250 mL IV fluid over 3 hours. 1, 3, 5
Mild to Moderate Hypomagnesemia (Oral Magnesium)
Start magnesium oxide 12 mmol (≈480 mg elemental magnesium) taken at night when intestinal transit is slowest for maximal absorption. 1, 2, 3 If serum magnesium remains low after 1–2 weeks, increase to 24 mmol daily (single or divided doses). 1, 2
Night-time dosing is essential because intestinal transit slows during sleep, allowing better absorption. 1, 2
Recheck Magnesium and Adjust
Recheck serum magnesium 2–3 weeks after starting supplementation or after any dose adjustment. 2 Once on stable dosing, monitor every 3 months. 2 In patients with short bowel syndrome, high gastrointestinal losses, or on medications affecting magnesium (diuretics, PPIs, calcineurin inhibitors), check more frequently—every 2 weeks initially. 2
When Oral Magnesium Fails: Refractory Cases
If oral supplementation fails to normalize levels after adequate trial (4–6 weeks at 24 mmol daily), add oral 1-alpha hydroxy-cholecalciferol starting at 0.25 μg daily, titrating up to 9 μg to improve magnesium balance. 1, 2, 3 Monitor serum calcium weekly to avoid hypercalcemia. 1, 2
For patients with short bowel syndrome, severe malabsorption, or high-output stomas where oral therapy is ineffective, use subcutaneous magnesium sulfate 4–12 mmol added to saline bags, administered 1–3 times weekly. 1, 2 Alternatively, consider IV magnesium sulfate. 1, 2
Most magnesium salts are poorly absorbed and may paradoxically worsen diarrhea or stomal output in patients with gastrointestinal disorders—this is why oral therapy often fails in these populations. 1, 2
Correct Associated Electrolyte Abnormalities in the Right Order
Never attempt to correct hypokalemia or hypocalcemia before normalizing magnesium—these abnormalities are completely refractory to supplementation until magnesium is corrected. 1, 2, 3 Hypomagnesemia causes dysfunction of multiple potassium transport systems and increases renal potassium excretion, making hypokalemia resistant to potassium treatment. 1, 2 Hypomagnesemia also impairs parathyroid hormone release, causing hypocalcemia that will not respond to calcium supplementation. 1, 2
- Correct volume depletion with IV saline
- Replete magnesium
- Then correct potassium and calcium
Calcium normalization typically occurs within 24–72 hours after magnesium repletion begins. 1
Renal Function: Absolute Contraindication
Check renal function before any magnesium supplementation. 2, 6 Magnesium is excreted solely by the kidneys, and supplementation is absolutely contraindicated when creatinine clearance <20 mL/min due to risk of life-threatening hypermagnesemia. 1, 2, 5 In severe renal insufficiency (CrCl <30 mL/min), maximum dose is 20 g over 48 hours with frequent serum monitoring. 1
The only exception is life-threatening torsades de pointes or cardiac arrest, where 1–2 g IV bolus may be given with extreme caution and close monitoring. 1, 2
Monitoring During IV Replacement
During IV magnesium administration, monitor continuously for signs of magnesium toxicity: 1
- Loss of patellar reflexes (occurs at ~4 mEq/L) 5
- Respiratory depression (at ~10 mEq/L) 5
- Hypotension and bradycardia 1, 5
- Heart block 5
Have calcium chloride or calcium gluconate immediately available to reverse magnesium toxicity if needed. 1 Effective anticonvulsant serum levels range from 2.5–7.5 mEq/L; levels >12 mEq/L may be fatal. 5
Special Populations Requiring Higher Doses
Patients with short bowel syndrome, jejunostomy, or severe malabsorption often require the higher dose of 24 mmol daily or parenteral supplementation because intestinal absorptive surface is reduced and each liter of jejunostomy output contains substantial magnesium (~100 mmol/L sodium plus proportionate magnesium). 1, 2, 7
For patients on continuous renal replacement therapy (CRRT), use dialysis solutions containing magnesium to prevent ongoing losses, as 60–65% of critically ill patients on CRRT develop hypomagnesemia. 1, 2 Regional citrate anticoagulation increases magnesium losses further. 1, 2
Common Pitfalls to Avoid
- Starting oral magnesium without correcting volume depletion first in patients with GI losses—secondary hyperaldosteronism will perpetuate renal magnesium wasting. 1, 2
- Attempting to correct hypokalemia before magnesium—potassium supplementation will fail until magnesium is normalized. 1, 2
- Assuming normal serum magnesium excludes deficiency—serum levels can be normal despite severe intracellular depletion. 3, 4
- Giving magnesium to patients with CrCl <20 mL/min outside of life-threatening emergencies—this causes fatal hypermagnesemia. 1, 2
- Using hypotonic oral fluids (tea, coffee, juices) in patients with jejunostomy—these cause sodium and magnesium loss from the gut. 2