What are the side effects of Magnesium sulfate (MgSO4)?

Side Effects of Magnesium Sulfate (MgSO4)

Magnesium sulfate commonly causes minor side effects like flushing and sweating, but can lead to serious toxicity including respiratory depression, cardiac conduction abnormalities, and neuromuscular blockade at higher serum levels. 1

Common Side Effects

• Flushing and sweating are among the most common minor side effects reported with parenteral magnesium administration 1
• Nausea and vomiting can occur in patients receiving magnesium therapy 2
• Hypotension may develop due to the vasodilatory effects of magnesium 1
• Injection site pain or irritation can occur, particularly with intramuscular administration 3

Dose-Dependent Toxicity Effects

Magnesium toxicity manifestations correlate with serum magnesium levels:

• Loss of deep tendon reflexes (particularly patellar reflex) occurs at serum magnesium levels of 3.5-5 mmol/L (4-6 mg/100mL) and is the earliest clinical sign of toxicity 1
• Respiratory depression (breathing rate <16/min) occurs at higher levels and is a serious concern at approximately 5-6.5 mmol/L 4
• Cardiac conduction abnormalities including:
  • Prolonged PR, QRS, and QT intervals occur at 2.5-5 mmol/L 2
  • AV nodal block and bradycardia at 6-10 mmol/L 2
  • Cardiac arrest can occur when magnesium levels exceed 12.5 mmol/L 4
• Neuromuscular blockade resulting in:
  • Muscle weakness 1
  • Flaccid paralysis 1
  • Hypothermia 1

Special Considerations

Renal Impairment

• Patients with renal impairment are at significantly higher risk of magnesium toxicity as magnesium is exclusively eliminated by the kidneys 1
• Dose adjustments are required for patients with renal dysfunction - geriatric patients should not exceed 20g in 48 hours due to common age-related renal impairment 1
• Urine output should be maintained at >100 mL over the four hours preceding each dose 1

Pregnancy and Fetal Considerations

• Continuous administration beyond 5-7 days can cause fetal hypocalcemia, skeletal demineralization, and osteopenia 1
• Neonatal fractures have been reported with prolonged maternal use 1
• Neonatal neuromuscular or respiratory depression may occur when administered close to delivery 1

Drug Interactions

• Additive CNS depression when combined with barbiturates, narcotics, anesthetics, or other CNS depressants 1
• Enhanced neuromuscular blockade when used with neuromuscular blocking agents 1
• Potential for heart block in digitalized patients if calcium is required to treat magnesium toxicity 1

Monitoring and Management of Toxicity

• Clinical monitoring is essential and should include:

  • Regular assessment of deep tendon reflexes (particularly patellar reflex) 1
  • Respiratory rate monitoring (should remain ≥16 breaths/minute) 1
  • Urine output measurement 1
  • Continuous cardiac monitoring for arrhythmias in cases of toxicity 2

• Treatment of magnesium toxicity:

  • Immediate administration of intravenous calcium (calcium chloride 10% 5-10 mL or calcium gluconate 10% 15-30 mL IV over 2-5 minutes) as a physiological antagonist 2
  • Discontinuation or delay of subsequent magnesium doses if toxicity signs develop 1
  • Vasopressors may be needed if hypotension persists despite calcium administration 2

Incidence of Adverse Effects

• The overall rate of absent patellar reflex in clinical studies was approximately 1.6% 3
• Respiratory depression occurred in about 1.3% of patients 3
• Delay in repeat administration due to side effects occurred in 3.6% of cases 3
• Need for calcium gluconate administration was less than 0.2% 3
• Mortality directly attributable to magnesium sulfate is extremely rare when properly monitored 3

When administering magnesium sulfate, the key to preventing serious toxicity is vigilant clinical monitoring of reflexes, respiratory rate, and urine output, with particular caution in patients with renal impairment or those receiving other CNS depressants 1, 2.