Magnesium Sulfate | Medication Reference

Bedside Snapshot

Core Indications (IV): Torsades de pointes, ventricular dysrhythmias in hypomagnesemia, AF/SVT adjunct, severe asthma, eclampsia/preeclampsia, moderate–severe hypomagnesemia, digoxin-induced dysrhythmias
Typical Adult Doses:
- Torsades (with pulse): 1–2 g IV over 5–15 min
- Torsades arrest: 1–2 g IV/IO over 1–2 min
- Life-threatening hypomagnesemia: 4 g total (2 g over 5–15 min, then 2 g over 30–60 min)
- Status asthmaticus: 2 g IV over 15–20 min
- Eclampsia: 4–6 g IV load over 20–30 min, then 1–2 g/h infusion
Pediatric: 25–50 mg/kg IV/IO over 10–20 min (max 2 g) for hypomagnesemia/torsades or status asthmaticus
Key Danger: Hypermagnesemia toxicity – loss of deep tendon reflexes, bradycardia/heart block, respiratory depression, hypotension (especially in renal failure or prolonged OB infusions); treat with IV calcium and stop magnesium
Special Notes: 1 g MgSO₄ ≈ 8 mEq Mg²⁺; 1 g = 2 mL of 50% solution; 50% solution is very hyperosmolar (~4,060 mOsm/L) and must be diluted before IV use

Brand & Generic Names

Generic Name: Magnesium sulfate (MgSO₄)
Brand Names: Generally available as generic magnesium sulfate injection; no major brand names in U.S. market

Medication Class

Electrolyte; anticonvulsant; antiarrhythmic; bronchodilator adjunct; tocolytic; vasodilator; electrolyte replenisher

Pharmacology

Mechanism of Action:

Physiologic calcium antagonist: Mg²⁺ competes with Ca²⁺ at voltage-gated calcium channels in myocardium, vascular smooth muscle, and neuromuscular junction → decreased Ca²⁺ influx, decreased SA/AV node automaticity and conduction, and vasodilation
Membrane stabilizer: Modulates Na⁺, K⁺, and Ca²⁺ flux and slows SA node impulse formation and AV conduction → raises action potential threshold and stabilizes excitable membranes (antiarrhythmic effect)
NMDA receptor block: Extracellular Mg²⁺ produces voltage-dependent block of NMDA receptors → less glutamate-mediated excitation, contributing to anticonvulsant and analgesic effects
Smooth muscle relaxation: Reduces acetylcholine release at the neuromuscular junction and inhibits myometrial action potentials, leading to bronchodilation and uterine relaxation (tocolytic effect)

Pharmacokinetics (IV – clinically relevant):

Onset: Immediate anticonvulsant/antiarrhythmic effect with IV dosing
Duration: ~30 minutes for anticonvulsant effect from single IV bolus; total-body repletion lasts longer depending on stores and renal function
Distribution: ~1–2% in extracellular fluid; significant uptake into bone and intracellular space; ~30% protein bound
Elimination: Almost entirely renal; accumulation and hypermagnesemia occurs in GFR <30 mL/min
Elemental content: 1 g magnesium sulfate ≈ 8 mEq (≈4 mmol) elemental Mg²⁺

Indications

Torsades de pointes / polymorphic VT associated with prolonged QT (with or without a pulse)
Ventricular dysrhythmias and seizures associated with life-threatening hypomagnesemia
Adjunct in atrial fibrillation/flutter and other supraventricular arrhythmias when hypomagnesemia suspected
Severe asthma / status asthmaticus as an adjunct to standard therapy
Prevention and treatment of eclamptic seizures in severe preeclampsia/eclampsia
Correction of moderate–severe hypomagnesemia when IV route preferred
Digitalis/digoxin-induced ventricular dysrhythmias when Digoxin immune Fab is unavailable

Dosing & Administration

Available IV Forms (typical):

Injection (for dilution): 50% solution = 500 mg/mL MgSO₄ heptahydrate
- Very hyperosmolar (~4,060 mOsm/L); must be diluted before IV use
Premixed IV bags: Commonly 1 g/100 mL, 2 g/100 mL (institution-dependent)

Rule of thumb:

1 g = 2 mL of 50% solution
1 g ≈ 8 mEq Mg²⁺

Adult Dosing (IV Only):

Always follow local protocols and institutional policies. Doses below are typical EM/ICU/ACLS ranges, not a standing order set.

Indication	Dose	Notes
Torsades de pointes / polymorphic VT (with pulse)	1–2 g IV in 50–100 mL D5W/NS over 5–15 min	May follow with 0.5–1 g/h infusion if recurrent; aligns with ACLS/emergency references
Torsades / suspected hypomagnesemic arrest (pulseless VT/VF)	1–2 g IV/IO diluted in 10–20 mL D5W/NS, given over ~1–2 min	Typically after initial defibrillation and epinephrine; avoid rapid undiluted IV in ROSC patients
Life-threatening hypomagnesemia (torsades, seizures, profound QT prolongation)	4 g total: 2 g IV over 5–15 min + 2 g IV over 30–60 min	Consider maintenance infusion 1–2 g/h (renal-adjusted)
Moderate–severe hypomagnesemia (non-emergent)	2–4 g IV over 2–4 h; or 4–8 g IV over 24 h	Slower infusion → better intracellular uptake and less renal wasting; reduce dose for GFR <30
AF/SVT in critically ill (suspected hypomagnesemia)	2–4 g IV over 10–30 min	Used empirically when Mg²⁺ low or unknown; may follow with infusion if AF persists
Status asthmaticus / severe asthma exacerbation (adjunct)	2 g IV in 50–100 mL over 15–20 min	Safe, modest benefit; should not delay other critical therapies
Severe preeclampsia/eclampsia (seizure prophylaxis/treatment)	Load: 4–6 g IV in 100 mL over 20–30 min; then 1–2 g/h IV	Continue ≥24 h after last seizure or delivery; follow OB service protocol for monitoring
Digitalis/digoxin-induced dysrhythmias (if Digoxin Fab unavailable)	1–2 g IV over 5 min, then 1 g/h infusion	Requires continuous ECG and frequent electrolytes; adjunct to standard digoxin toxicity management

Maximums / Special Situations:

Severe renal impairment (GFR <30): Common practice ≤20 g over 48 h total and avoid repeated boluses without levels
OB (preeclampsia/eclampsia): Many protocols cap daily dose around 30–40 g/day; use lower totals with renal dysfunction

Pediatric Dosing (IV Only):

Always defer to PALS and local pediatric/OB guidelines.

Hypomagnesemia or torsades de pointes (PALS): 25–50 mg/kg IV/IO over 10–20 min; max 2 g/dose
Status asthmaticus (adjunct): 25–75 mg/kg IV over 15–20 min, commonly 25–50 mg/kg; max 2 g
Pediatric acute nephritis / non-emergent hypomagnesemia: 25–50 mg/kg IV/IM q4–6h for 3–4 doses as needed; max 2 g/dose

Administration:

Dilution: 50% (500 mg/mL) solution is very hyperosmolar (~4,060 mOsm/L) – dilute to typical concentrations like 1–2 g in 50–100 mL before IV use
Rate:
- Arrest: 1–2 g IV/IO over ~1–2 minutes
- Non-arrest bolus: 1–2 g over ≥5–15 minutes
- Larger doses (≥2 g) over 20–60 minutes or more to limit hypotension and flushing
Line/compatibility: Prefer a running IV line; avoid mixing with other negative inotropes/vasodilators (e.g., CCBs) in the same line when possible
Monitoring: Continuous ECG for arrhythmia/eclampsia therapy; frequent vitals, respiratory status, and deep tendon reflexes (especially OB and high-dose infusions); for prolonged therapy monitor Mg²⁺, Ca²⁺, K⁺, and creatinine

Contraindications

Absolute Contraindications:

Known hypersensitivity to magnesium sulfate
Heart block (2nd or 3rd degree) or significant baseline AV conduction delay in unpaced patients
Myasthenia gravis (risk of myasthenic crisis)
Diabetic coma (per some package inserts)

Major Precautions:

Renal failure (GFR <30 mL/min): Markedly increased risk of hypermagnesemia; initial "loading" is usually okay with careful monitoring, but repeated boluses require levels and dose reduction
Neuromuscular disease / NMB use: Magnesium potentiates nondepolarizing and depolarizing neuromuscular blockers and can unmask or worsen myasthenia gravis → respiratory failure
OB tocolysis >5–7 days: Prolonged high-dose OB use has been associated with fetal bone demineralization, neonatal hypocalcemia, and fractures – avoid long-term tocolytic use
Concomitant CCB or beta-agonist tocolytics: Increased risk of pulmonary edema and hypotension with nifedipine or terbutaline; many OB protocols prohibit these combinations

Renal Function Critical: With normal kidneys, magnesium is extremely forgiving. With GFR <30, it behaves more like a narrow-therapeutic-index drug – watch levels and reflexes closely.

Adverse Effects

Common / Dose-Related (usually transient):

Flushing, warmth, sweating
Nausea, vomiting
Mild hypotension or lightheadedness
Injection-site irritation, especially if concentrated and peripheral

Signs of Impending Toxicity (Hypermagnesemia):

Depressed or absent deep tendon reflexes
Increasing somnolence, generalized weakness
Bradycardia, PR/QRS prolongation, heart block, QT prolongation
Respiratory depression or apnea
Hypotension and vasodilation
In OB: fetal bradycardia, neonatal hypotonia/respiratory depression

Approximate Serum Mg and Clinical Correlates:

4–7 mEq/L: Therapeutic range for eclampsia
8–10 mEq/L: Loss of deep tendon reflexes
10–15 mEq/L: Respiratory depression/paralysis
≥25–30 mEq/L: Cardiac arrest

Management of Toxicity: Stop magnesium immediately; give IV calcium (e.g., calcium gluconate 1–2 g IV over 5–10 min); support airway/ventilation; consider dialysis in severe renal failure or massive overdose.

Drug Interactions

Neuromuscular blockers (depolarizing and non-depolarizing): Magnesium potentiates neuromuscular blockade → increased risk of prolonged paralysis or apnea, especially in OB and ICU patients
Calcium-channel blockers (e.g., nifedipine, verapamil, diltiazem): Additive negative inotropy/chronotropy and vasodilation → hypotension, heart block risk with high-dose magnesium
Digitalis (digoxin): Magnesium is used to treat some digoxin-induced dysrhythmias, but electrolyte shifts and conduction effects require close ECG and K⁺/Mg²⁺ monitoring
Oral chelation (tetracyclines, etc.): Mainly an issue for oral magnesium (reduced absorption of other drugs via chelation); relevant if transitioning from IV to PO

Special Populations

Renal Impairment:

GFR <30 mL/min: Markedly increased risk of accumulation and toxicity
Initial loading dose usually acceptable with close monitoring
Reduce maintenance doses and frequency; monitor serum magnesium levels closely
Common practice: limit to ≤20 g over 48 hours

Hepatic Impairment:

No specific dose adjustment required
Exercise caution in patients with both hepatic and renal dysfunction

Pregnancy & Lactation:

Pregnancy: Category D (FDA) – used for eclampsia/preeclampsia despite risks
Standard of care for eclampsia seizure prophylaxis and treatment
Prolonged use (>5–7 days) associated with fetal bone demineralization
Monitor for neonatal hypotonia, respiratory depression, and hypocalcemia
Lactation: Excreted in breast milk; generally considered compatible with breastfeeding at therapeutic doses

Pediatric Considerations:

Dosing: 25–50 mg/kg IV/IO (max 2 g per dose)
Use PALS guidelines for torsades and status asthmaticus
Monitor carefully for signs of toxicity

Geriatric Considerations:

Increased risk of renal impairment – assess GFR before dosing
May be more sensitive to CNS and cardiovascular effects
Monitor deep tendon reflexes, respiratory status, and vitals closely

Monitoring

Clinical Monitoring:

Continuous ECG monitoring during arrhythmia or eclampsia therapy
Vital signs: blood pressure, heart rate, respiratory rate
Deep tendon reflexes (especially in OB patients and high-dose infusions)
Respiratory status and oxygen saturation
Urine output (especially in OB protocols)
Level of consciousness and neuromuscular function

Laboratory Monitoring:

Serum magnesium levels (especially with prolonged therapy, renal impairment, or signs of toxicity)
Serum calcium, potassium
Renal function (creatinine, GFR) before and during therapy
In OB: continuous fetal monitoring when applicable

Clinical Pearls

Arrhythmias & Torsades: In torsades with prolonged QT, magnesium is first-line even when the serum Mg²⁺ is "normal" – the effect is electrophysiologic, not just replacement. For recurrent torsades, a continuous infusion (0.5–1 g/h) is often more effective and smoother than repeated small boluses.

Hypomagnesemia is Common and Often Missed: Hypomagnesemia is very common in critically ill patients and frequently coexists with hypokalemia and arrhythmias. If you're chasing refractory hypokalemia or AF in a sick patient, it's almost always reasonable to check Mg and often to give Mg empirically.

Renal Function is the Main Safety Limiter: With normal kidneys, magnesium is extremely forgiving. With GFR <30, it behaves more like a narrow-therapeutic-index drug – watch levels and reflexes closely.

Asthma: IV magnesium is safe, cheap, and quick; benefit is modest but side-effects are light. Great for "we've done all the right things and they're still tight" – just don't let it delay escalation to NIV/intubation when indicated.

OB Practice: For eclampsia, magnesium is the drug of choice for seizure prophylaxis/treatment and is superior to phenytoin/benzodiazepines for preventing recurrent seizures. OB protocols usually hard-wire: Mg dose, DTR checks, RR thresholds, urine-output cutoffs, and "give Ca gluconate now" triggers – follow those religiously.

Documentation: Document total grams given, renal function, relevant ECG changes, and any concurrent CCBs/NMBs. When in doubt, fall back to your local protocol / medical direction – especially for AF Mg infusions and OB regimens.

References

1. Hicks, M. A., & Tyagi, A. (2023). Magnesium sulfate. In StatPearls. StatPearls Publishing. Retrieved November 14, 2025, from https://www.ncbi.nlm.nih.gov/books/
2. Drugs.com. (2024). Magnesium sulfate monograph for professionals. Retrieved November 14, 2025, from https://www.drugs.com/
3. Medscape. (n.d.). MgSO4 (magnesium sulfate) – dosing, indications, interactions, adverse effects. Retrieved November 14, 2025, from https://reference.medscape.com/
4. Farkas, J. (2024). Hypomagnesemia. Internet Book of Critical Care (EMCrit Project). Retrieved November 14, 2025.
5. Farkas, J. (2023). Critical asthma exacerbation. Internet Book of Critical Care (EMCrit Project). Retrieved November 14, 2025.
6. Farkas, J. (2023). Atrial fibrillation (AF) & flutter complicating critical illness. Internet Book of Critical Care (EMCrit Project). Retrieved November 14, 2025.
7. WebMD LLC. (2025). Magnesium (antidote) – drug monograph. Medscape. Retrieved November 14, 2025.
8. RxList. (2021). Magnesium sulfate: Side effects, uses, dosage, interactions, warnings. RxList. Retrieved November 14, 2025, from https://www.rxlist.com/
9. DrugBank Online. (n.d.). Magnesium sulfate (DB00653). Retrieved November 14, 2025, from https://go.drugbank.com/