How the kidneys handle magnesium
Magnesium is the fourth most abundant mineral in the body and is essential for muscle function, nerve conduction, blood pressure regulation and bone strength. Around 95% of filtered magnesium is reabsorbed in the renal tubules; the kidneys then excrete the surplus in urine. As CKD progresses and eGFR falls below 30 ml/min/1.73 m², magnesium excretion slows and serum levels rise. By CKD stage 5 and on dialysis, mild hypermagnesaemia is the rule rather than the exception.
Hidden magnesium in everyday products
The most common cause of dangerous hypermagnesaemia in CKD is NOT diet — it is over-the-counter products patients (and sometimes clinicians) forget contain magnesium: • LAXATIVES — Milk of Magnesia (magnesium hydroxide), magnesium citrate sachets, Epsom salts (magnesium sulphate) taken orally • ANTACIDS — Maalox, Mylanta, Rennie Spearmint, Gaviscon Double Action • SUPPLEMENTS — magnesium oxide, glycinate, malate, threonate, taurate (often sold for sleep, anxiety, cramps or migraine) • 'CALM' AND 'SLEEP' BLENDS — many combine 200–400 mg magnesium with melatonin or L-theanine • EPSOM SALT BATHS — small amounts absorbed through skin; usually harmless but worth flagging in advanced CKD If you are on dialysis or CKD stage 4–5, none of these should be used without renal team approval.
Symptoms of hypermagnesaemia
Magnesium toxicity progresses predictably with serum level: • 1.1–1.5 mmol/L — usually silent • 1.5–2.0 mmol/L — flushing, nausea, drowsiness, muscle weakness, low blood pressure • 2.0–3.0 mmol/L — loss of deep tendon reflexes, slow heart rate, ECG changes • > 3.0 mmol/L — respiratory depression, complete heart block, cardiac arrest Because early symptoms are non-specific, blood monitoring is the only reliable way to detect rising magnesium in CKD. Severe hypermagnesaemia is treated with IV calcium gluconate and, in advanced CKD, dialysis.
Safe laxative and antacid choices in CKD
For constipation in CKD, safer first-line options include: • Lactulose — not absorbed, no electrolyte load • Macrogol/polyethylene glycol (Movicol, Laxido) — renal-safe and well tolerated • Senna — short courses are fine • Bisacodyl (Dulcolax) — short courses For reflux or indigestion, safer antacid choices include: • Calcium-based antacids in moderation (but watch calcium load in advanced CKD) • Alginate-based products (Gaviscon Original) — check labels for magnesium • Proton pump inhibitors (omeprazole, lansoprazole) prescribed by your GP — short courses preferred as long-term PPI use has been linked to kidney injury Always check labels and ask your pharmacist to confirm a product is magnesium-free.
Magnesium in food
Magnesium-rich foods include wholegrains, nuts, seeds, dark leafy greens, beans and dark chocolate. Many of these are already limited in CKD for other reasons — nuts and seeds for potassium and phosphate, wholegrains for phosphate. Normal portions of food are not the problem; concentrated supplements and laxatives are. See our companion article on magnesium-rich foods for portion-by-portion guidance.
When to seek urgent help
If you accidentally take a large dose of a magnesium product (e.g. a full bottle of Milk of Magnesia) and have CKD, contact NHS 111 or your renal team straight away. Symptoms of severe hypermagnesaemia — profound weakness, slow or irregular pulse, difficulty breathing — are a 999 emergency.
