Causes of Elevated Magnesium (Hypermagnesemia)
Hypermagnesemia most commonly results from excessive magnesium intake (particularly magnesium-containing laxatives or antacids) in the setting of impaired renal excretion, though it can occur even with normal kidney function when large amounts are ingested or retained in the gastrointestinal tract. 1, 2, 3
Primary Mechanisms Leading to Hypermagnesemia
Excessive Magnesium Intake
Magnesium-containing laxatives and antacids are the most frequent culprits, particularly:
- Magnesium hydroxide (Milk of Magnesia) used for constipation 2, 3
- Magnesium oxide preparations 1
- Magnesium sulfate salts 1
Critical pitfall: In patients with constipation or bowel obstruction, retained magnesium-based laxatives in the gut serve as a continuous reservoir for magnesium absorption, leading to progressive toxicity even after discontinuation 3, 4. This explains why hypermagnesemia can be fatal despite normal renal function initially.
Iatrogenic magnesium administration includes:
- Magnesium sulfate for preterm labor in pregnant women 1
- Renacidin (magnesium-containing solution) for struvite stone dissolution 5
- Excessive intravenous magnesium supplementation 6
Impaired Renal Excretion
Renal dysfunction is the most important predisposing factor:
- Acute kidney injury 5
- Chronic kidney disease/end-stage renal disease 2, 4
- Creatinine clearance <20 mL/min represents absolute contraindication to magnesium supplementation 6, 7
The kidneys normally excrete excess magnesium efficiently, so even massive oral intake is typically well-tolerated with normal renal function. However, when kidney function is compromised, even therapeutic doses can accumulate to toxic levels 1, 8.
High-Risk Clinical Scenarios
Elderly patients with multiple risk factors are particularly vulnerable:
- Concurrent renal impairment (even mild) 4, 8
- Chronic constipation requiring regular laxative use 2, 3
- Bowel disorders (sigmoid volvulus, intestinal obstruction) that prolong magnesium contact time 4
- Polypharmacy with over-the-counter magnesium products 3, 4
Patients on continuous renal replacement therapy (CRRT) can develop hypermagnesemia when:
- Dialysis solutions contain magnesium 7
- Regional citrate anticoagulation is used (chelates ionized magnesium) 7
Clinical Manifestations by Severity
Moderate hypermagnesemia (2.5-5 mmol/L or approximately 6-12 mg/dL) presents with:
- Prolonged PR, QRS, and QT intervals on ECG 1
- Muscular weakness 6
- Drowsiness and confusion 6
- Vasodilation and hypotension 6
Severe hypermagnesemia (6-10 mmol/L or >12 mg/dL) causes:
- Atrioventricular nodal conduction block 1
- Bradycardia 1
- Complete paralysis 5
- Respiratory depression 6
- Cardiac arrest 1, 2
- Coma 8
Prevention Strategies
Absolute contraindications to magnesium-containing preparations include 6:
- Acute or chronic kidney disease with creatinine clearance <20 mL/min
- Bowel obstruction or severe constipation
- Elderly patients with multiple comorbidities requiring close monitoring
When magnesium supplementation is necessary, the European Society of Medical Oncology recommends 1:
- Ensuring adequate renal function before initiation
- Avoiding prolonged use of magnesium-containing laxatives
- Regular monitoring of serum magnesium levels, especially in high-risk populations 2, 8
Critical monitoring points: Healthcare providers must recognize that serum magnesium levels can rise precipitously even after stopping magnesium intake if gastrointestinal retention persists, and that mortality remains high even with prompt dialysis when levels exceed 10 mg/dL 2, 3.