ClinCalc Pro
Menu
Calculators Drugs Pathways AI Dx Donate Get the App
Bronchodilator / Smooth muscle relaxant Pregnancy: Compatible — widely used in pre-eclampsia/eclampsia at higher doses; single bronchodilator dose safe. Monitor neonatal magnesium if used near delivery.

Magnesium Sulphate IV (Acute Asthma)

Brand names: Magnesium Sulphate Injection 50% (2 mmol/mL)

Adult dose

Dose: 1.2–2 g IV over 20 minutes
Route: Intravenous infusion (dilute in 100 mL sodium chloride 0.9% or glucose 5%)
Frequency: Single dose (may repeat once in life-threatening asthma on specialist advice)
Max: 2 g per dose
Acute severe or life-threatening asthma unresponsive to initial bronchodilator therapy. BTS/SIGN: give 1.2–2 g IV over 20 min in adults with acute severe asthma (SpO₂ <92%, unable to complete sentences, RR >25, HR >110). MAGICS trial supports use. Onset of bronchodilation within 10–20 min. Mechanism: blocks calcium-mediated smooth muscle contraction and inhibits mast cell degranulation.

Paediatric dose

Dose: 40 mg/kg
Route: IV infusion over 20 minutes
Frequency: Single dose
Max: 2 g (2000 mg)
Children: 40 mg/kg (max 2 g) IV over 20 minutes. Licensed and recommended by BTS/SIGN for acute severe asthma in children unresponsive to initial nebulised bronchodilators.

Dose adjustments

Renal

Use with extreme caution in renal impairment — magnesium renally excreted; risk of hypermagnesaemia. Monitor serum magnesium.

Hepatic

No dose adjustment required.

Paediatric weight-based calculator

Children: 40 mg/kg (max 2 g) IV over 20 minutes. Licensed and recommended by BTS/SIGN for acute severe asthma in children unresponsive to initial nebulised bronchodilators.

Clinical pearls

  • MAGICS trial (Goodacre et al, Lancet 2013): IV magnesium significantly improves lung function and reduces hospital admissions in acute severe asthma — now standard practice
  • BTS/SIGN 2023: recommended for adults and children with acute severe asthma (SpO₂ <92%) after initial treatment with salbutamol + ipratropium + IV/IM hydrocortisone
  • Infuse over exactly 20 minutes — faster infusion causes hypotension and flushing
  • Calcium gluconate is the antidote for magnesium toxicity: 10 mL of 10% calcium gluconate IV
  • Inhaled magnesium sulphate (nebulised isotonic Mg 250 mg in 2.5 mL): some evidence as adjunct to salbutamol nebulisers in children

Contraindications

  • Severe renal impairment (eGFR <30 mL/min) — relative contraindication; use only if benefit clearly outweighs risk
  • Hypermagnesaemia
  • Heart block (relative)

Side effects

  • Flushing and warmth (common during infusion)
  • Nausea
  • Hypotension (if infused too rapidly)
  • Bradycardia and respiratory depression (toxic levels >5 mmol/L)
  • Loss of tendon reflexes (early sign of toxicity — check patellar reflex)

Interactions

  • Calcium channel blockers — additive hypotension
  • Neuromuscular blocking agents — magnesium potentiates neuromuscular blockade
  • Aminoglycosides — additive neuromuscular effects

Monitoring

  • SpO₂ and peak flow before and after infusion
  • BP and HR during infusion
  • Serum magnesium if renal impairment or repeat dose
  • Patellar tendon reflex (absent = toxicity)

Reference: BNFc; BTS/SIGN Asthma Guideline 2023; MAGICS Trial (Goodacre et al, Lancet 2013); NICE BNF 84. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

Calculators
Pathways