Pharmacology · Electrolytes and rehydration solutions

Magnesium sulfate !

IV or IM magnesium to prevent and control eclamptic seizures and to replace low magnesium. A high-alert medicine: excess stops breathing.

Updated 2026-11-01

Class: Electrolytes and rehydration solutions
Target: Calcium entry at the nerve terminal and NMDA-type channels (no single receptor)
ATC code: B05XA05
WHO Model List of Essential Medicines

Mechanism of action

Magnesium competes with calcium at the motor nerve terminal and reduces the release of acetylcholine, which depresses neuromuscular transmission. It also depresses the central nervous system and relaxes vascular smooth muscle. These effects prevent and control the seizures of pre-eclampsia and eclampsia. High levels cause loss of reflexes, respiratory paralysis and heart block.

Magnesium competes with calcium at the nerve terminal
Less acetylcholine is released at the end plate
Neuromuscular and CNS excitability fall: seizures are prevented
Excess: reflexes vanish, breathing and the heart are depressed

Pharmacokinetics

Absorption: Given IV or IM; IM levels take longer to reach effect
Elimination: Excreted only by the kidneys, in proportion to the filtration rate
Onset and duration: IV: anticonvulsant action almost immediate, lasting about 30 minutes; IM: about one hour, lasting three to four hours

Indications

Prevention and control of seizures in pre-eclampsia and eclampsia Preeclampsia →
Magnesium deficiency (hypomagnesaemia) when replacement by vein is needed

Adverse effects

Common

Flushing, sweating and a feeling of warmth
Low blood pressure
Drowsiness and weakness
Pain at the injection site (IM)

Contraindications

Heart block or myocardial damage (parenteral use)

Key interactions

Opioids, barbiturates, sedatives or anaesthetics: Additive CNS depression
Neuromuscular blocking agents: Excessive and prolonged neuromuscular block
Digoxin and other cardiac glycosides: Serious conduction changes and heart block, especially if calcium is needed to treat toxicity

What is monitored

Patellar (deep tendon) reflexes before each dose
Respiratory rate and oxygen saturation
Urine output
Serum magnesium
Blood pressure, heart rate and, in labour, the fetal heart rate

Administration and safety

High-alert medicine: use a programmed infusion pump and an independent double check of the product, concentration and pump settings.
The concentrated solution must be diluted before IV use; premixed bags lower the risk of mistakes.
Use one standard product per unit where possible; store concentrated vials away from patient-care areas.
Calcium gluconate (the antidote) and equipment to support breathing are kept at the bedside.

Nursing care

Before each dose and regularly during infusion: patellar reflex, respiratory rate and urine output; stop the infusion and call if reflexes disappear, breathing slows or urine output falls.
Check serum magnesium as ordered, more often in kidney failure.
Watch for flushing, drowsiness, slurred speech or muscle weakness as early toxicity signs.
In pregnancy: monitor the fetal heart rate and tell the neonatal team, because the newborn can have low muscle tone and breathing.
Keep calcium gluconate ready at the bedside.

Pregnancy and breastfeeding

The medicine of choice to prevent and treat eclamptic seizures; continuous use for more than about a week can cause low calcium and bone problems in the fetus.

Test yourself

Think of the answer, then tap the question to see it.

How does magnesium sulfate prevent eclamptic seizures?

It competes with calcium at the nerve terminal, reducing acetylcholine release and neuromuscular excitability, and depresses the central nervous system.

Which three checks come before each dose of magnesium sulfate?

Patellar reflexes, respiratory rate and urine output (magnesium is excreted only by the kidneys).

What is the antidote for magnesium toxicity?

IV calcium (such as calcium gluconate), with breathing support if needed.

Which drug class has its effect prolonged by magnesium?

Neuromuscular blocking agents: magnesium adds to their block.

Add to my cards (4)

Preeclampsia

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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: DailyMed - MAGNESIUM SULFATE- magnesium sulfate heptahydrate injection, solution (DailyMed (U.S. National Library of Medicine), 2026); DailyMed - MAGNESIUM SULFATE- magnesium sulfate heptahydrate injection, solution (DailyMed (U.S. National Library of Medicine), 2025); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026); eEML - Electronic Essential Medicines List (World Health Organization, 2025).

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