Obstetrics & Gynaecology/Magnesium sulfate

Magnesium sulfate

also: MgSO4 · magnesium

Overview

First-line for eclamptic seizures and seizure prophylaxis in severe pre-eclampsia; also fetal neuroprotection in early preterm birth. Narrow safety margin — monitor for toxicity.

Mechanism

Acts as a membrane stabiliser and NMDA-receptor antagonist (anticonvulsant), causes cerebral vasodilation, and competes with calcium at the neuromuscular junction (smooth-muscle relaxation, tocolytic-like effect).

Indications

Eclampsia — treatment of seizures (first-line, NOT diazepam/phenytoin)
Severe pre-eclampsia — seizure prophylaxis
Fetal neuroprotection (<30–34 weeks, imminent birth)
Severe/life-threatening asthma; torsades de pointes

Adverse effects

Loss of deep tendon reflexesclassic

EARLIEST sign of toxicity — check patellar reflexes during infusion.

Respiratory depressionserious

Later toxicity; monitor respiratory rate.

Cardiac arrest / hypotensionserious

Flushing, warmth, nauseacommon

Cautions & contraindications

Myasthenia gravisall

Worsens neuromuscular blockade.

Significant renal impairmentrenal

Accumulation → toxicity; reduce dose, monitor levels.

Interactions

Additive with calcium-channel blockers (hypotension) and neuromuscular blockers

Monitoring & kinetics

Deep tendon reflexes, respiratory rate, urine output (and Mg²⁺ levels if renal impairment/toxicity),Antidote = IV calcium gluconate

IV loading + maintenance infusion. Renally cleared — caution in oliguria.

Choosing it

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Source: NICE NG133 — Hypertension in pregnancy · BNF — Magnesium sulfate