Synopsis
Recognise bidirectional magnesium toxicity, correct linked calcium and potassium disturbance, and adapt replacement or removal to renal function.
- Serum magnesium represents a small extracellular fraction, so a result can underestimate tissue depletion; interpret symptoms, losses and companion electrolytes as well as the number.
- Hypomagnesaemia causes neuromuscular irritability, seizure, QT-related ventricular arrhythmia and renal potassium wasting, and it impairs PTH release and action to produce hypocalcaemia.
- Common low-magnesium drivers include diarrhoea, malabsorption, alcohol dependence, refeeding, diuretics, proton pump inhibitors, aminoglycosides, cisplatin, calcineurin inhibitors and uncontrolled diabetes.
Key red flags
Tremor, fasciculation, cramps, hyperreflexia, carpopedal spasm, nystagmus, confusion or seizure reflects increased neuromuscular excitability and companion calcium disturbance.
Investigation priorities
Confirm direction and severity and determine whether excretion can support replacement or recovery.
Management branches
Magnesium is low with symptoms, arrhythmia, linked electrolyte disturbance or an ongoing high-risk cause.
- Assess ABCDE, ECG, renal function, potassium and calcium, identify ongoing gastrointestinal or renal losses and stop or modify causal medicines where clinically safe.
- Use a locally selected oral preparation for stable mild disease; use monitored intravenous magnesium sulfate when severe symptoms, torsades, seizure or malabsorption makes enteral treatment inappropriate.
Key medicines
Magnesium sulfate for severe deficiencySelect concentration, dose and infusion duration from the current licensed product and local hypomagnesaemia chart according to symptoms, measured magnesium, weight, ongoing loss and renal function; serious arrhythmia uses its dedicated resuscitation protocol.
Oral magnesium preparationChoose the locally formulary-approved licensed salt and divided dose from the BNF or SmPC, titrating to tolerance, serum response and continuing losses rather than converting different salts by tablet count.