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Magnesium salt / bronchodilator / antiarrhythmic Pregnancy: See obstetrics entry (eclampsia). For asthma in pregnancy — use as per standard dosing.

Magnesium Sulphate IV (Additional Indications)

Brand names: Magnesium Sulphate Injection

Adult dose

Dose: Acute severe asthma: 2 g IV over 20 min (single dose); Torsades de Pointes: 2 g IV over 5–10 min
Route: IV infusion
Frequency: Single dose (asthma); repeat once (TdP) if needed
Max: 8 g/day
Severe asthma (not responding to standard treatment): 2 g MgSO₄ IV over 20 min (BTS Grade A recommendation). Torsades de Pointes: 2 g IV over 5–10 min; repeat once if needed. Note: magnesium sulphate for eclampsia/pre-eclampsia is a separate indication (4 g loading then 1 g/hour) — see OB/GYN drugs.

Paediatric dose

Dose: 40 mg/kg
Route: IV
Frequency: Single dose
Max: 2 g
Concentration: 500 mg/ml
Severe acute asthma: 40 mg/kg (max 2 g) IV over 20 minutes. BTS/SIGN evidence base: single IV dose reduces hospital admission in children with acute asthma. Nebulised MgSO4 (isotonic 150 mg/mL) also used as adjunct.

Dose adjustments

Renal

Reduce dose in renal impairment (accumulation — check deep tendon reflexes, respiratory rate, UO).

Hepatic

No dose adjustment required.

Paediatric weight-based calculator

Severe acute asthma: 40 mg/kg (max 2 g) IV over 20 minutes. BTS/SIGN evidence base: single IV dose reduces hospital admission in children with acute asthma. Nebulised MgSO4 (isotonic 150 mg/mL) also used as adjunct.

Clinical pearls

  • BTS: single dose IV MgSO4 (2 g over 20 min) in severe acute asthma reduces need for ICU admission
  • Toxicity monitoring: loss of patellar reflex (at ~3.5 mmol/L) precedes respiratory depression (>5 mmol/L)
  • Antidote for toxicity: calcium gluconate 10 mL of 10% IV given over 10 minutes
  • TdP: correct any precipitating hypokalaemia and hypomagnesaemia simultaneously
  • Nebulised isotonic MgSO4 (Magan study): used as adjunct with nebulised salbutamol

Contraindications

  • Hypermagnesaemia
  • Renal failure (use with extreme caution)

Side effects

  • Flushing and warmth (IV infusion)
  • Hypotension
  • Loss of deep tendon reflexes (toxicity — reflexes lost before respiratory arrest)
  • Respiratory depression (toxic — >5 mmol/L)
  • Cardiac arrest (very high levels)

Interactions

  • Calcium gluconate — reversal agent for magnesium toxicity (10 mL 10% IV)
  • Neuromuscular blockers — potentiates block (reduce doses)
  • Digoxin — enhanced toxicity in hypermagnesaemia

Monitoring

  • BP and HR (every 15 min during infusion)
  • Patellar reflex check
  • Respiratory rate and SpO2
  • Urine output

Reference: BNFc; BNF; BTS/SIGN Asthma Guideline 2023; Resuscitation Council TdP Algorithm. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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