Electrolyte / antiarrhythmic / anticonvulsant Pregnancy: As eclampsia may be life-threatening to mother and baby, magnesium sulfate may be administered in this condition. Magnesium crosses the placenta and may produce hypotonia, hyporeflexia and hypotension; given during labour it may cause respiratory depression of the newborn. Fetal heart rate should be monitored and use within 2 hours of delivery avoided. Continuous administration for more than 5 to 7 days in pregnancy can cause fetal hypocalcaemia, skeletal demineralisation and osteopenia. Breastfeeding: safety not established — not advisable unless considered essential.

Magnesium sulfate

Used in: Pre-eclampsia & Obstetric Emergencies

An intravenous magnesium salt used in emergency and critical care settings, notably for eclampsia and severe pre-eclampsia, life-threatening arrhythmias such as torsades de pointes, and as adjunctive therapy in severe acute asthma.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: Treatment of magnesium deficiency in hypomagnesaemia: up to 40 g, i.e. 160 mmol of magnesium ions (400 mL of a 10% solution), by slow intravenous infusion in glucose 5% over up to 5 days, may be required to replace the deficit (allowing for urinary losses)

Route: Intravenous infusion (slow intravenous infusion, diluted in glucose 5%; see SPC section 6.6 for dilution instructions)

Frequency: Dosage should be tailored to the individual patient's needs and responses. Plasma magnesium concentrations should be measured to determine the rate and duration of infusion and should be monitored throughout therapy. The replacement course may be given over up to 5 days

Max: Hypomagnesaemia replacement: up to 40 g (160 mmol of magnesium ions) in total. In renal impairment the dosage should not exceed 20 g in 48 hours (200 mL of a 10% solution, 80 mmol of magnesium ions)

Source is the UK SPC for Magnesium Sulfate 10% w/v solution for infusion; all millilitre figures quoted refer to the 10% solution and must be recalculated for other strengths. PREVENTION OF RECURRENT SEIZURES IN ECLAMPSIA (SPC §4.2, verbatim figures): a loading dose of 4 g (16 mmol) of magnesium ions IV (40 mL of a 10% solution), or in some cases 5 g (20 mmol) of magnesium ions IV (50 mL of a 10% solution), given over 5-15 minutes, is followed by an infusion of 1 g (4 mmol)/hour (10 mL of a 10% solution) continued for 24 hours after the last fit. RECURRENT CONVULSIONS: if convulsions recur, a further 2-4 g (8-16 mmol) of magnesium ions (20-40 mL of a 10% solution, depending on the woman's weight; 2 g (8 mmol) if less than 70 kg) is given IV over 5 minutes. Appropriate reductions in dosage should be made for patients with renal impairment; a suggested dose reduction in severe renal impairment is a maximum of 20 g (80 mmol of magnesium ions) over 48 hours. ELDERLY: no special recommendation; use with caution due to the risk of renal impairment in this age group. PAEDIATRIC POPULATION: the SPC states 'No special recommendation' — no paediatric regimen is given, so paedDose is null; verify paediatric use against a children's formulary. US labelling cross-check (Magnesium Sulfate in 5% Dextrose Injection, for eclampsia/pre-eclampsia): loading dose 4 to 6 grams over 15 minutes followed by a maintenance dosage of 1 to 2 grams every hour via infusion pump; maximum recommended dosage 30 to 40 grams over 24 hours; administration beyond 5 to 7 days is not recommended; in severe renal impairment and/or urine output less than 0.5 mL/kg/hour, 4 g loading dose then 1 g every hour, not exceeding 20 grams over 48 hours.

Dose adjustments

Renal

Doses must be reduced in renal impairment; caution must be observed to prevent exceeding the renal excretory capacity. The dosage should not exceed 20 g in 48 hours (200 mL of a 10% solution, 80 mmol of magnesium ions). Contraindicated in renal failure. Especially in patients with impaired renal function, magnesium may accumulate sufficiently to produce toxic effects.

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC; US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

Hypersensitivity to magnesium and its salts
Renal failure
Hepatic encephalopathy, hepatic failure
Parenteral magnesium salts should generally be avoided in patients with heart block
US labelling additionally lists heart block or myocardial damage, diabetic coma and myasthenia gravis as contraindications

Side effects

Respiratory depression; nausea, vomiting, drowsiness, confusion, coma, slurred speech, double vision (features of hypermagnesaemia)
Cardiac arrhythmias, cardiac arrest, ECG changes (prolonged PR, QRS and QT intervals), bradycardia
Flushing of the skin and hypotension due to peripheral vasodilatation
Loss of tendon reflexes due to neuromuscular blockade, muscle weakness
Electrolyte/fluid abnormalities (hypophosphataemia, hyperosmolar dehydration); isolated reports of maternal and fetal hypocalcaemia with high doses; hypersensitivity reactions; thirst

Interactions

Non-depolarising muscle relaxants (e.g. tubocurarine) — effect enhanced by parenteral magnesium salts; parenteral magnesium may enhance the effects of neuromuscular blocking agents generally
Aminoglycoside antibacterials — neuromuscular blocking effects may be additive
Nifedipine — profound hypotension was produced in two women given oral nifedipine
Digitalis glycosides — magnesium salts should be administered with caution to patients receiving them
CNS depressants (barbiturates, opiates, general anaesthetics) — additive central depressant effects; dosage of these agents must be carefully adjusted
Intravenous calcium antagonises the effects of magnesium; the muscle-stimulating effects of barium toxicity are reduced by magnesium

Clinical monograph

How it works

Magnesium acts as a physiological calcium antagonist and membrane stabiliser, modulating neuromuscular transmission, smooth muscle tone and cardiac conduction.

Prescribing in practice

Magnesium toxicity is the key hazard: monitor for loss of patellar reflexes, respiratory depression and hypotension, and have intravenous calcium available as the antidote.
Use lower doses and increased monitoring in renal impairment, where magnesium accumulates and toxicity develops more readily.
Administer intravenous doses at the recommended controlled rate, as rapid injection can cause flushing, hypotension and cardiac effects.

Monitoring

Monitor deep tendon reflexes, respiratory rate, blood pressure and urine output during therapy, with serum magnesium levels in prolonged or high-dose use or renal impairment.

Counselling the patient

Warmth, flushing or a feeling of heat during the infusion is common and usually settles.
Tell staff at once if you feel very weak, drowsy, breathless or unwell during treatment.

Evidence & guidelines

The Magpie trial established intravenous magnesium sulfate for eclampsia prevention in pre-eclampsia, and it is recommended in NICE and resuscitation guidance for eclampsia and torsades de pointes.

Reference: NICE NG133; UK Resus Council; BTS/SIGN asthma; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Calculators

Pathways

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