Electrolyte · Antiarrhythmic · Antiepileptic · Bronchodilator
Magnesium sulfate (MgSO4)
Also called Mag, MgSO4
Indications
- Seizure prevention and treatment in severe preeclampsia and eclampsia
- Hypomagnesemia, including with arrhythmias or refractory hypokalemia or hypocalcemia
- Torsades de pointes (off-label)
- Severe asthma not responding to initial treatment (off-label)
- Fetal neuroprotection before 32 weeks (off-label)
- AF rate-control adjunct and postoperative AF prevention (off-label)
Dosing
Infusion start: Severe preeclampsia or eclampsia: 4–6 g over 15–20 minutes, then 1–2 g/h. Titration: By reflexes, breathing, urine output and levels.
| Where | Dose |
|---|---|
| Cardiac arrest | Magnesium: 1–2 g IV/IO over 5–20 min for torsades |
| Rhythm control | Magnesium: 2 g over 15 min for polymorphic VT |
| Metabolic and electrolyte boluses | Magnesium sulfate: 1–2 g over 15–60 min (4–6 g load for preeclampsia, then 1–2 g/h); avoid in myasthenia gravis (can trigger crisis) |
| Unit conversions | Magnesium: 1 mEq/L = 0.5 mmol/L = 1.2 mg/dL |
| Unit conversions | Magnesium sulfate: 1 g = 8.1 mEq |
| Other ICU drugs | Magnesium, potassium, phosphate replacement: Smaller doses and closer levels |
On the quick cards
Mechanism of action
Magnesium acts as a physiologic calcium antagonist: it competes with Ca²⁺ at voltage-gated L-type channels in smooth and cardiac muscle and at presynaptic nerve terminals, so less Ca²⁺ enters. At the neuromuscular junction this cuts acetylcholine release; in the brain Mg²⁺ blocks the NMDA receptor channel and relaxes cerebral vessels, the likely basis of its effect in eclampsia. In the heart it suppresses the Ca²⁺-dependent early afterdepolarizations that trigger torsades without shortening the QT; as a Na⁺/K⁺-ATPase cofactor it helps cells hold K⁺.
Pharmacology
- CNS: prevents eclamptic seizures better than diazepam or phenytoin (Collaborative Eclampsia Trial 1995); it does not treat severe hypertension
- Neuromuscular: weakness and loss of deep tendon reflexes as levels rise; potentiates all neuromuscular blockers
- Cardiovascular: vasodilation and flushing with fast boluses; slows SA and AV conduction; outside the Vaughan Williams classes
- Respiratory: bronchodilation; respiratory depression at toxic levels
- Obstetric: relaxes the uterus; given before 32 weeks it lowered cerebral palsy (BEAM, Rouse 2008)
- Metabolic: needed for PTH release and to stop renal K⁺ wasting
Pharmacokinetics
| Onset | IV immediate; IM about 1 h (label) |
|---|---|
| Duration | About 30 min after IV and 3–4 h after IM (label), hence the infusion |
| Half-life | About 4–5 h in preeclampsia with normal kidneys; much longer in kidney failure |
| Distribution | Extracellular, then into cells and bone; Vd 14–49 L in pregnancy (Okusanya 2016); crosses the placenta |
| Protein binding | About 30%, mainly to albumin |
| Metabolism | None |
| Elimination | Kidneys only, in proportion to plasma level and GFR (label) |
| Target levels | Eclampsia prophylaxis about 4.8–8.4 mg/dL (4–7 mEq/L) in common protocols; the label gives 2.5–7.5 mEq/L |
| Dialysis | Removed by hemodialysis |
Chemistry
- Magnesium sulfate heptahydrate (MgSO₄·7H₂O, Epsom salt, MW 246.5): a fully dissociated inorganic salt, so pKa, lipid solubility and stereochemistry do not apply
- 1 g supplies about 98 mg (8.1 mEq, 4.06 mmol) of elemental magnesium
- 50% vials (4 mEq/mL, about 4,060 mOsm/L, pH 5.5–7.0) must be diluted to 20% or less for IV use (label)
- Premixed 4% bags (20 g/500 mL, 40 g/1000 mL) reduce mixing errors; a high-alert drug with an independent double-check
Adverse effects and cautions
In the one-pagers
- Obstetric EmergenciesPreeclampsia and eclampsia: Eclampsia: new seizures with no other cause. Call for help, protect from injury, side-lying, prevent aspiration, oxygen. Most stop on their own; magnesium prevents the next.
- Pharmacology by Class and SuffixAntidotes and reversal: Magnesium sulfate: Calcium gluconate
- Rhythm GalleryVentricular: Torsades: polymorphic VT twisting around the baseline, usually with a long QT. Pulseless: defibrillate. With a pulse: magnesium 1–2 g IV over about 15 min; avoid QT-prolonging drugs such as amiodarone.
- DKA and HHSInsulin deficiency, counter-regulation and ketones: DKA begins when insulin is too low to restrain liver and fat while glucagon, catecholamines, cortisol and growth hormone rise (2024 consensus). The liver makes glucose from glycogen, amino acids, lactate and glycerol while muscle and fat take up less. Above the renal threshold (about 180–200 mg/dL) osmotic diuresis drains water, sodium, potassium
- Fluids, IV Therapy and ElectrolytesElectrolyte replacement: label safety points: Magnesium sulfate: safe dosing shows a patellar reflex and RR of about 16 or more; keep urine at least 100 mL every 4 hours. Reflexes weaken above about 4 mEq/L and may vanish near 10, with risk of respiratory paralysis. Support breathing and give IV calcium for toxicity.
- Mental Health: Meds and CommunicationAlcohol Withdrawal and CIWA-Ar: Thiamine prevents Wernicke encephalopathy (confusion, ataxia, eye signs). Seizure precautions; check magnesium.
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In the algorithms
Other electrolyte drugs
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Sources
- FDA label: Magnesium sulfate 50% (2025)
- ACOG Practice Bulletin 222 (2020)
- Okusanya 2016 (pharmacokinetics review)
- Iseri 1984 (magnesium as a calcium blocker)
For education and exam preparation only. Follow your institution’s protocols. Typical adult values for study. Follow the order, the label and your institution's protocols.