Low magnesium keeps potassium and calcium low — replace all three.
Severe (under 0.4 mmol/L) or symptomatic (arrhythmia, seizures, tetany): IV replacement.
Act now
- 1
ECG: long QT; torsades de pointes needs urgent IV magnesium.
Assess
- Causes: PPIs, diuretics, alcohol, diarrhoea, refeeding, poor intake, aminoglycosides, cisplatin.
- Renal impairment: replace cautiously and recheck sooner.
Investigations
- Magnesium, K⁺, calcium, phosphate, U&E.
Manage
- Mild to moderate (0.4–0.7 mmol/L) and able to swallow: oral magnesium. Diarrhoea is common.
- Severe or symptomatic: IV magnesium sulfate, recheck the next day.
- Review PPIs and diuretics.
Escalate when
- Arrhythmia, seizures or tetany: senior now.
Doses and prescribing
Magnesium aspartate (Magnaspartate)
10 mmol sachet oral 1–2 times daily.
Magnesium sulfate
20 mmol (5 g) IV in 250 mL 0.9% sodium chloride over 2–4 hours, per local policy; repeat as needed.
Magnesium sulfate
2 g (8 mmol) IV over 10 minutes for torsades de pointes.
Check the BNF and your local formulary before prescribing.
Written to current guidance · sources checked 14 Nov 2026. We watch NICE for updates weekly.
Sources (1)
Reviewed by Aboodee Alhasso, MPharm MRCPharm IP on 14 Nov 2026
For healthcare professionals, as an aid to clinical judgement — not a substitute for it. Follow your local policy and ask a senior when in doubt.