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[Pharmacology](https://durgical.com/en/pharmacology/) · [Electrolytes and rehydration solutions](https://durgical.com/en/pharmacology/classes/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](https://durgical.com/en/pharmacology/classes/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

## ⚠ High-alert medication

An error with this medicine can cause serious harm: double checks and protocols are used.

## 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 →](https://durgical.com/en/conditions/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)

### Serious (rare, important)

- Magnesium toxicity: loss of reflexes, flaccid paralysis, respiratory depression and arrest
- Heart block and circulatory collapse
- Hypocalcaemia with tetany
- Newborn toxicity (weak muscles, depressed breathing) after continuous infusion before delivery
- Fetal low calcium and bone abnormalities with continuous use for longer than about a week

## 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.

## Memory aid

Magnesium «mutes» the nerve: first the knee goes quiet, then the breath; check the reflex before every dose and keep calcium as the «volume knob».

## 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)](https://durgical.app/estudiar/?add=/en/pharmacology/magnesium-sulfate/)

## Related drugs

- [Calcium gluconate](https://durgical.com/en/pharmacology/calcium-gluconate/)
- [Potassium chloride](https://durgical.com/en/pharmacology/potassium-chloride/)

## Related conditions

- [Preeclampsia](https://durgical.com/en/conditions/preeclampsia/)

[← Back to the class: Electrolytes and rehydration solutions](https://durgical.com/en/pharmacology/classes/electrolytes-and-rehydration-solutions/)

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](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=e921a46b-9a5b-4163-9c88-224247541f14) (DailyMed (U.S. National Library of Medicine), 2026); [DailyMed - MAGNESIUM SULFATE- magnesium sulfate heptahydrate injection, solution](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=a5a9f565-639c-4b22-b9e5-718bac7cfcf3) (DailyMed (U.S. National Library of Medicine), 2025); [ATCDDD - ATC/DDD Index](https://atcddd.fhi.no/atc_ddd_index/?code=B05XA05) (WHO Collaborating Centre for Drug Statistics Methodology, 2026); [eEML - Electronic Essential Medicines List](https://list.essentialmeds.org/) (World Health Organization, 2025).

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Representative model, for educational purposes only.

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