⚠️ Caution: For peripheral administration, magnesium sulfate 50% must be diluted to ≤5% (20 mmol in 100 mL) to avoid pain, thrombophlebitis, or tissue necrosis. Use central access if higher concentrations are required.

💊 About

Magnesium sulfate is a highly soluble inorganic salt used in emergency medicine, obstetrics, and critical care.
About 60% of body magnesium is stored in bone; the remainder is intracellular - essential for enzyme function, ATP metabolism, and cell membrane stability.
Always check the BNF entry for updated dosing guidance.

⚙️ Mechanism of Action

Acts as a physiological calcium antagonist - blocks calcium influx at voltage-gated channels, relaxing smooth muscle.
Stabilises excitable membranes, reducing neuronal excitability and arrhythmogenicity.
Modulates NMDA receptors in the CNS, producing anticonvulsant and neuroprotective effects.
Acts as a cofactor in >300 enzymatic reactions including ATP synthesis and energy metabolism.

🎯 Indications

🌬️ Acute severe asthma (FEV₁ < 25% predicted).
❤️ Torsades de Pointes and other ventricular arrhythmias.
💧 Hypomagnesaemia (especially in malnutrition, alcoholism, diuretics).
🤰 Pre-eclampsia & eclampsia – prevention and treatment of seizures (RCOG 2023).
🧠 Neuroprotection in imminent preterm birth (24–29 weeks gestation) per NICE NG25.

💉 Parenteral Doses (Magnesium sulfate 1 g = 4 mmol)

Indication	Dose	Infusion Rate	Comments
Torsades de Pointes	8 mmol (2 g) IV	Over 10–15 min in 100 mL 5% Dextrose / NaCl	Then 72 mmol (18 g) / 24 h infusion if recurrent.
Acute Severe Asthma	8 mmol (2 g) IV	Over 20 min	Single dose; improves FEV₁ and reduces admission risk.
Eclampsia	16 mmol (4 g) loading → 96–192 mmol (12–24 g) / 24 h	Loading over 20 min; maintenance infusion 1 g / h	Use Pritchard or Zuspan regimens; monitor reflexes, RR, and urine output.
Hypomagnesaemia	20 mmol (5 g)	Over 1–3 h	Max 50 mmol / day; up to 160 mmol over 5 days if persistent deficiency.

💉 IM Injection

Used only if IV access unavailable; inject deep IM (buttock or thigh).
Small risk of pain and sterile abscess formation - use caution.

🧮 Oral Replacement

Replace orally if serum Mg < 0.7 mmol/L and symptoms present (e.g. tremor, arrhythmia, weakness).
Magnesium glycerophosphate: 4 mmol per tablet, 2 tabs TDS (≈ 24 mmol daily).
GI side effect: diarrhoea - titrate to tolerance.

⚕️ Interactions & Precautions

Prolongs the action of non-depolarising muscle relaxants - monitor in anaesthesia or ITU.
Potentiates CNS depression when combined with benzodiazepines or opioids.
Adjust dose in renal impairment - accumulation leads to respiratory depression or cardiac arrest.

🚫 Contraindications

Severe renal failure - do not exceed 20 g / 48 h.
Myasthenia gravis (may worsen weakness).
Heart block or profound hypotension.

⚠️ Adverse Effects

🩸 Hypotension (especially with rapid bolus).
💤 CNS depression and loss of reflexes with excessive serum levels.
🫁 Respiratory depression in overdose.
🧠 Flushing, warmth, and sedation are early signs of rising magnesium levels.

☠️ Magnesium Toxicity

Therapeutic range: 2–3.5 mmol/L. Toxicity > 5 mmol/L → respiratory depression, cardiac arrest.
❌ Do not continue loading rates beyond the recommended period.
Max bolus: 6 g over 15–20 min; maintenance ≤ 3 g / h.
Antidote: 10 mL of 10% calcium gluconate IV over 10 min → repeat as required.
Monitor RR > 12 / min, urine > 100 mL / 4 h, and preserved patellar reflexes during infusion.

📚 References

BNF: Magnesium Sulfate
ICID Salisbury: IV Magnesium Sulfate Guidelines
RCHT Hypomagnesaemia Guideline (NHS Cornwall)
RCOG Green-top 10(A): Pre-Eclampsia and Eclampsia (2023)
NICE NG25: Preterm Labour and Birth (2022)

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Magnesium Sulphate - Sulfate