Reasoning GP · Algorithm · 1-page summary
Magnesium deficiency in adults
For: adults in primary care with serum magnesium <0.7 mmol/L (lab range), or low K⁺/Ca²⁺ that will not correct. Not for: children; hospital IV replacement (full algorithm, step 6); obstetric magnesium sulfate for pre-eclampsia (obstetric team); refeeding (same-day hospital, NICE CG32).
Core rule: potassium and calcium will not stay corrected while magnesium is low — check magnesium whenever K⁺ or Ca²⁺ is refractory and replace it first. IV magnesium is a hospital treatment only; review any long-term PPI.
0Stop — call 999 now — regardless of the exact magnesium value
- Torsades, ventricular or haemodynamically significant arrhythmia, or significant QT prolongation → 999
- Seizure, tetany or severe neuromuscular symptoms; muscle weakness with dysphagia or respiratory muscle involvement → 999
- Syncope or collapse with suspected electrolyte-related arrhythmia → 999
- Never in general practice: IV magnesium (hospital, with ECG, BP, renal and electrolyte monitoring) or IM magnesium. Torsades follows the resuscitation algorithm.
1Triage by level — confirm; repeat if unexpected
| Mg mmol/L | Action |
|---|---|
| <0.5 (severe), or symptomatic | Same-day hospital assessment, even if apparently asymptomatic; IV in hospital (SPS) |
| ~0.5–0.7, stable, eGFR ≥30 | Oral replacement in primary care + treat the cause (block 5) |
2Screen — any one moves the patient out
- Refractory low K⁺ or Ca²⁺ with low Mg → immediate Same-day hospital
- AKI, eGFR <30, dialysis, transplant or oliguria (magnesium accumulates) → Same-day hospital; renal advice before any replacement.
- Cannot take or absorb oral treatment → Same-day hospital
- Suspected or high-risk refeeding → Same-day hospital/dietetic assessment (NICE CG32); not an isolated Mg plan.
- On cisplatin, amphotericin or aminoglycosides → replacement directed by the prescribing specialist team; liaise.
3Assess — once blocks 0–2 are answered
☐ Cramps, tremor, weakness, paraesthesiae, palpitations; Chvostek/Trousseau ☐ ECG if Mg substantially low, symptoms, low K⁺, QT-prolonging drugs or cardiac disease (long PR/QT, wide QRS) ☐ U&E/eGFR before any replacement, K⁺, adjusted Ca²⁺, phosphate, albumin; glucose/HbA1c if relevant ☐ Medicines, alcohol, diarrhoea, laxatives, nutrition; coeliac serology if malabsorption suspected
Not in primary care: IV/IM magnesium; urinary Mg or FEMg is a specialist-level test.
4Classify the cause — where is it being lost?
- PPI (prolonged use; impaired gut absorption) → review indication and dose.
- Renal loss — loop or thiazide diuretics, ciclosporin, tacrolimus, cisplatin, aminoglycosides, amphotericin, foscarnet; marked glycosuria; aldosteronism; Gitelman/Bartter.
- GI loss — chronic diarrhoea, IBD, coeliac, laxatives, short bowel, post-bariatric → treat the cause.
- Alcohol or poor intake → check K⁺, phosphate, Ca²⁺, glucose, nutrition; thiamine per protocol (Pabrinex has no Mg).
- Shift — refeeding (block 2), hungry bone, acute pancreatitis. Uncertain or recurrent → block 6.
5Treat today — ~0.5–0.7, stable, eGFR ≥30
- Oral Mg (local formulary): magnesium aspartate 10 mmol sachets or glycerophosphate 4 mmol chewable 1–2 TDS (not if GFR <30 or high phosphate). Start low, divided, with food; diarrhoea limits dose. Doses: NICE/SPC, BNF, local formulary.
- PPI: lowest effective dose; step down or stop if the indication allows. Must continue (Barrett's, severe oesophagitis, high-risk gastroprotection) → oral Mg + periodic Mg.
- Diuretic: lowest effective dose; do not switch loop to thiazide just for Mg. Amiloride only with specialist advice (hyperkalaemia).
- Recheck Mg, K⁺, Ca²⁺, renal function about 7 days; 1–4 weeks if substantially low or high dose; then 3–6-monthly if the cause persists.
6Refer routinely
- Nephrology: unexplained, persistent, recurrent or familial; low K⁺ with metabolic alkalosis (Gitelman/Bartter); renal wasting not explained by drugs.
- Gastroenterology: uninvestigated malabsorption, post-bariatric. Endocrinology: primary aldosteronism.
- Low despite tolerated oral Mg and PPI cannot stop → gastroenterology, renal or acute medicine. Pregnancy → with the obstetric team.
7Never reassure by — risk depends on level, symptoms, ECG, K⁺/Ca²⁺ and renal function
- Calling K⁺ or Ca²⁺ "refractory" before magnesium is checked and replaced.
- Treating a level below 0.5 as routine oral replacement.
- Starting oral magnesium before checking eGFR.
- A "therapeutic" digoxin level when Mg is low — check digoxin and Mg together.
- Adding a QT-prolonging drug before low Mg is corrected.
- Assuming a long-term PPI still has an indication.
8Safety-net — say it, then document it
999 “Call 999 if you have palpitations with dizziness, fainting or collapse, a fit, painful muscle spasms, or new weakness with difficulty swallowing or breathing.”
Same day: “Contact us today if you cannot take or keep down the magnesium. If it gives you diarrhoea, take a lower dose. Come for your blood test in about a week.” Remote: Mg <0.5 or refractory low K⁺/Ca²⁺ needs same-day assessment, not a message.
✓Safe to manage in primary care only if — all five, documented
No block 0–2 feature; Mg ~0.5–0.7, stable
eGFR ≥30; ECG done where indicated and normal
K⁺, Ca²⁺, phosphate checked
Cause acted on (PPI, diuretic, alcohol, gut); recheck booked
Safety-net given
Summary of the full Reasoning GP magnesium deficiency algorithm, which holds the detail, hospital IV replacement and evidence. Sources: NHS SPS Treating acute hypomagnesaemia in adults; MHRA Drug Safety Update (2012) PPIs and hypomagnesaemia; NICE CG184, CG32; ERC 2025; local formulary; SPCs. Clinical reference — follow local acute-transfer, renal and prescribing policies and check doses: NICE/SPC, BNF.