MAGNESIUM SULFATE
Clinical safety rating
cautionComprehensive clinical and safety monograph for MAGNESIUM SULFATE (MAGNESIUM SULFATE).
Magnesium sulfate acts as a physiological calcium channel blocker. It inhibits calcium influx into presynaptic nerve terminals, reducing acetylcholine release at the neuromuscular junction and decreasing muscle contraction. It also antagonizes NMDA receptors and stabilizes neuronal membranes.
| Metabolism | Magnesium sulfate is not metabolized. It is primarily excreted unchanged by the kidneys via glomerular filtration. |
| Excretion | Primarily renal (90-95% as unchanged drug); minor biliary/fecal (<5%) |
| Half-life | Terminal elimination half-life approximately 4-6 hours in patients with normal renal function; prolonged to 12-24 hours or more in renal impairment, necessitating dose adjustment |
| Protein binding | Approximately 30-35%, primarily to albumin |
| Volume of Distribution | 0.5-0.7 L/kg; distributes primarily to extracellular fluid, with lower penetration into cells and CSF |
| Bioavailability | Intravenous: 100%; intramuscular: approximately 80-90%; oral: 10-30% due to limited gastrointestinal absorption and osmotic effects |
| Onset of Action | Intravenous: immediate to 1 minute; intramuscular: 15-30 minutes; oral: 1-2 hours (but not used for anticonvulsant effect orally) |
| Duration of Action | Intravenous: 30 minutes to 1 hour (for anticonvulsant effect); intramuscular: 3-4 hours; clinical effect duration is dose-dependent and influenced by renal clearance |
| Molecular Weight | 120.37 |
IV: Loading dose 4-6 g over 20-30 minutes, followed by maintenance infusion 1-2 g/hour for seizure prophylaxis in severe preeclampsia/eclampsia. IM: 4-8 g deep IM initially, then 4 g every 4 hours as needed.
| Dosage form | INJECTABLE |
| Renal impairment | GFR <20 mL/min: Use with extreme caution; reduce dose by 50% and monitor serum magnesium levels. GFR 20-50 mL/min: Reduce dose by 25-50%. Avoid if anuric. |
| Liver impairment | No specific Child-Pugh based adjustments; however, caution in severe hepatic impairment due to risk of hypotension and CNS depression. |
| Pediatric use | Neonates (seizures): IV 0.2-0.4 mL/kg of 50% solution (100-200 mg/kg) over 5-10 minutes; may repeat once. Children: 0.2-0.4 mL/kg IV/IM (as 50% solution) for hypomagnesemia; maximum single dose 8 g. |
| Geriatric use | Start at low end of dosing range due to age-related renal function decline; frequent monitoring of serum magnesium and clinical status recommended. |
| 1st trimester | Magnesium sulfate crosses the placenta. Use only if clearly needed; risk of skeletal abnormalities if continuous infusion >5-7 days. |
| 2nd trimester | Use only if clearly needed; monitor fetal growth with prolonged use. |
| 3rd trimester | FDA indication for prevention of eclampsia; may cause neonatal respiratory depression or hypotonia if administered close to delivery. |
Clinical note
Comprehensive clinical and safety monograph for MAGNESIUM SULFATE (MAGNESIUM SULFATE).
| Placental transfer | Magnesium sulfate readily crosses the placenta, achieving fetal serum levels similar to maternal levels. |
| Breastfeeding | Magnesium sulfate is excreted into breast milk in small amounts. Levels are low and considered compatible with breastfeeding. Monitor infant for hypotonia or sedation if high maternal doses used. |
| Lactation Rating | L2 (Safer) |
| Teratogenic Risk | FDA Pregnancy Category D (prolonged use >5-7 days or continuous use near term). First trimester: Limited data; no evidence of major malformations. Second/third trimester: Prolonged use may cause fetal hypocalcemia, skeletal demineralization, and neonatal hypotonia. Use >5-7 days near term increases risk of neonatal hypermagnesemia (respiratory depression, hypotonia). Transient fetal bradycardia may occur during IV administration. |
| Fetal Monitoring | Monitor maternal vital signs, deep tendon reflexes (loss indicates magnesium toxicity), respiratory rate (≥12/min), urine output (≥100 mL every 4 hours), serum magnesium levels (therapeutic 4-7 mEq/L). Fetal/neonatal: Continuous fetal heart rate monitoring, assess for fetal bradycardia, neonatal hypermagnesemia. Monitor calcium levels if prolonged therapy. |
| Fertility Effects | No known adverse effects on fertility in humans. Animal studies show no impairment of fertility at standard doses. High doses may cause hypotonia in pregnant animals, but no direct effect on fertility indices. |
■ FDA Black Box Warning
Continuous infusion of magnesium sulfate in pregnant women beyond 5-7 days may lead to fetal hypocalcemia, skeletal abnormalities, and osteopenia.
| Serious Effects |
Heart block or myocardial damageSevere renal impairment (creatinine clearance <20 mL/min) due to risk of hypermagnesemiaHypersensitivity to magnesium sulfateMaternal myasthenia gravis
| Precautions | Use with caution in patients with renal impairment due to risk of hypermagnesemia. Monitor serum magnesium, respiratory rate, deep tendon reflexes, and urine output during continuous administration. May cause hypotension, respiratory depression, and cardiac arrest in overdose. |
| Food/Dietary | No significant food interactions. Oral magnesium supplements may be taken with meals to minimize gastrointestinal discomfort. Avoid excessive intake of high-fiber foods, which can reduce magnesium absorption. |
| Clinical Pearls | Monitor deep tendon reflexes, respiratory rate, and urine output during IV infusion. Have calcium gluconate ready to reverse magnesium toxicity. In preeclampsia/eclampsia, administer IV loading dose of 4-6 g followed by 1-2 g/hour. Do not exceed 5 g IM per injection site. Use with caution in renal impairment; reduce dose if creatinine clearance <30 mL/min. Check serum magnesium levels if toxicity suspected. Avoid concurrent use with CNS depressants. |
| Patient Advice | Report symptoms of magnesium toxicity such as muscle weakness, slowed breathing, or lightheadedness. · Do not drive or operate heavy machinery until you know how magnesium sulfate affects you. · Take oral magnesium supplements with food to reduce stomach upset. · Inform your doctor if you have kidney problems or are taking other medications. · During infusion, you may feel warmth, flushing, or sweating. |
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