Magnesium Sulfate

Approved Indications:

• Eclampsia and Severe Preeclampsia: Prevention and control of seizures during pregnancy and postpartum.
• Hypomagnesemia: Treatment of acute or chronic magnesium deficiency.
• Torsades de Pointes: Management of this specific form of polymorphic ventricular tachycardia.
• Barium Poisoning: Used to reduce gastrointestinal absorption of barium salts.
• Constipation: Occasional short-term use as an osmotic laxative (oral form).
• Bowel Evacuation: As part of bowel preparation prior to diagnostic procedures or surgery.

Clinically Accepted Off-Label Uses:

• Acute Severe Asthma (Adjunctive): Intravenous magnesium sulfate may be used to reduce bronchospasm in severe asthma unresponsive to initial treatments.
• Fetal Neuroprotection: Administered to women at risk of preterm delivery (<32 weeks) to reduce the risk of cerebral palsy in the newborn.
• Digoxin-Induced Arrhythmias: Used in selected cases of ventricular arrhythmias associated with digitalis toxicity.

Routes of Administration: Intravenous (IV), Intramuscular (IM), Oral

Adults:

• Eclampsia/Preeclampsia (Seizure Prevention):
• Loading dose: 4–6 g IV over 15–20 minutes
• Maintenance dose: 1–2 g/hour IV infusion for 24–48 hours after the last seizure or delivery
• Hypomagnesemia:
• Mild: 1 g (8 mEq) IV or IM every 6 hours for 4 doses
• Severe: 4–8 g IV over 2–4 hours or as continuous infusion, adjusted by serum levels
• Torsades de Pointes:
• 1–2 g IV over 5–15 minutes; may repeat as necessary
• Acute Severe Asthma:
• 1.2–2 g IV infused over 20 minutes as a single dose
• Oral Laxative Use:
• 5–10 g dissolved in 250 mL of water, once daily as needed

Pediatrics:

• Hypomagnesemia:
• 25–50 mg/kg/dose IV every 6 hours; maximum single dose: 2 g
• Seizure Prophylaxis or Neuroprotection (Obstetric Use):
• Same maternal dosing as adults; fetal benefits achieved transplacentally
• Acute Severe Asthma:
• 25–75 mg/kg IV over 20 minutes; maximum dose: 2 g

Geriatric Patients:

• Use adult dosing with caution; initiate at the lower end of the dosage range due to decreased renal function. Monitor serum magnesium and renal function regularly.

Special Populations:

• Renal Impairment:
• Dose must be reduced or avoided due to the risk of accumulation and magnesium toxicity.
• Frequent monitoring of serum magnesium is essential.
• Hepatic Impairment:
• No dose adjustment typically required.

Administration Notes:

• IV doses should be diluted and infused slowly.
• Monitor reflexes, respiratory rate, urine output, and serum magnesium levels.
• For oral use, dissolve powder in water and consume immediately.

Magnesium sulfate acts as a central nervous system depressant, neuromuscular blocker, and calcium antagonist. It reduces acetylcholine release at the neuromuscular junction and decreases neuronal excitability, thereby preventing or controlling seizures in preeclampsia and eclampsia. In cardiac arrhythmias such as torsades de pointes, magnesium stabilizes myocardial cell membranes and prolongs the refractory period. In bronchial smooth muscle, magnesium inhibits calcium influx, promoting muscle relaxation in severe asthma. As an osmotic laxative, it retains water in the intestines, increasing peristalsis and promoting bowel evacuation.

• Absorption:
• Oral: Poor systemic absorption; primarily local gastrointestinal effect
• IV/IM: Rapid and complete systemic absorption
• Onset of Action:
• IV: Immediate
• IM: 1 hour
• Oral: 30 minutes to 6 hours
• Bioavailability:
• IV: 100%
• Oral: Low bioavailability
• Distribution:
• Widely distributed in extracellular fluids and tissues
• Crosses the placenta and is excreted in breast milk
• Metabolism:
• Not metabolized
• Half-life:
• 4–5 hours; may be prolonged in renal impairment
• Elimination:
• Primarily excreted unchanged via the kidneys
• Clearance correlates with glomerular filtration rate

• Pregnancy:
• FDA Pregnancy Category D (when used long-term or at high doses)
• Safe and commonly used short-term for eclampsia prevention and fetal neuroprotection with appropriate monitoring
• Lactation:
• Small amounts are excreted into breast milk
• Generally considered safe; no significant adverse effects reported in nursing infants
• Caution:
• Avoid extended use beyond 5–7 days due to risk of fetal bone changes; short-term use is supported by clinical guidelines

• Primary Class: Mineral and Electrolyte Supplement / Anticonvulsant
• Subclass: Antiarrhythmic Agent (Class V), Tocolytic, Osmotic Laxative (oral)

• Known hypersensitivity to magnesium sulfate or any component of the formulation
• Severe renal impairment or anuria
• Myocardial damage or recent heart block
• Persistently elevated magnesium levels (hypermagnesemia)
• Bowel obstruction (if given orally as a laxative)

• High-Risk Populations:
• Patients with impaired renal function, neuromuscular diseases (e.g., myasthenia gravis), or existing electrolyte imbalances
• Magnesium Toxicity:
• Signs include loss of deep tendon reflexes, respiratory depression, bradycardia, and cardiac arrest
• Monitor serum magnesium, respiration, urine output, and reflexes regularly
• Antidote:
• Calcium gluconate should be readily available for magnesium overdose
• Long-Term Use in Pregnancy:
• Prolonged use may cause fetal bone demineralization and should be avoided

Common:

• Neurological: Flushing, warmth, sedation, headache
• Gastrointestinal (oral use): Diarrhea, cramping, nausea
• Injection Site: Pain, swelling (IM)

Serious (Dose-dependent):

• Respiratory: Respiratory depression, apnea
• Cardiovascular: Hypotension, bradycardia, cardiac arrhythmias
• Neuromuscular: Loss of reflexes, muscle weakness, paralysis
• Electrolyte Imbalance: Hypermagnesemia, hypocalcemia

Onset & Severity:

• Rapid with IV use; toxicity is more likely with high doses, prolonged infusion, or impaired renal function

• Calcium Channel Blockers (e.g., nifedipine): Increased risk of hypotension and neuromuscular blockade
• Neuromuscular Blockers (e.g., vecuronium): Enhanced neuromuscular blockade
• CNS Depressants: Additive sedative and respiratory depression effects
• Digoxin: Interferes with cardiac conduction
• Tetracyclines: Reduced absorption if taken concurrently (oral route)

Enzyme Systems:

• Not metabolized by CYP450 enzymes; interactions are primarily pharmacodynamic

• FDA: Continues to advise against long-term (>5–7 days) use in pregnancy due to risk of fetal bone changes
• WHO & ACOG: Recommend magnesium sulfate as the first-line agent for seizure prevention and fetal neuroprotection in preterm labor
• Clinical Practice Updates: Reinforce the need for monitoring magnesium levels and limiting use to essential clinical scenarios under supervision

• Temperature: Store at 20°C to 25°C (68°F to 77°F); excursions permitted to 15°C–30°C
• Humidity: Store in a dry place, away from moisture
• Light: Protect from direct light
• Handling Instructions:
• For injection: Inspect solution for particles or discoloration
• Do not freeze
• For oral powder: Reconstitute with water and consume immediately